Bill Ackman

Ackman is a concentrated, long-hold, activist investor. He typically owns 5–10 positions at a time — sometimes fewer.

Each one involves exhaustive research. If he's buying, he has usually already built a 100-slide deck explaining exactly what's wrong with the company and exactly what needs to change to fix it.

His style is the opposite of index investing. He wants a controlling voice at the table.

He wants to talk to the CEO. He wants the board to change.

That's the "activist" part — he's not just buying a stock and hoping it goes up. He's buying it with a plan to force the thing that will make it go up.

The strategy works until it doesn't. When the thesis is right, he wins massively.

When the thesis is wrong and he's also very public about it, the losses are spectacular.

David Einhorn

Einhorn is a long/short fundamental investor. On the long side, he looks for cheap, misunderstood businesses where the market has gotten the story wrong.

On the short side, he looks for companies with accounting irregularities, unsustainable business models, or management teams making misleading claims. His edge historically was doing the detailed work that others skipped: actually reading the footnotes, actually questioning the numbers, actually calling the bluff in public.

He goes public with his short theses, which most short-sellers avoid. He presents at conferences.

He names names. This is controversial — critics say it''s market manipulation, supporters say it''s journalism.

The Allied Capital and Lehman Brothers situations suggest it''s at least occasionally correct.

Bill Ackman

Ackman's core belief is that markets frequently misprice assets when the story around them is either too negative or too complicated. He looks for businesses he can understand deeply, with a gap between what the market thinks they're worth and what he thinks they're worth.

He's talked extensively about asymmetric bets — situations where you can be wrong and lose a small amount, but right and make a lot. He believes activist investing works because most company boards are too comfortable and most CEOs have too little accountability.

He thinks public pressure, when backed by real analysis, is a legitimate tool for creating value. Whether that makes him a hero or a villain depends on which company you ask.

David Einhorn

Einhorn believes the market frequently misprices securities because most investors are lazy or short-term focused. His philosophy is that detailed, patient fundamental research creates an edge — not access, not algorithms, not macro calls.

He also believes that companies with accounting problems eventually get found out. He has framed his public short campaigns as a form of market policing: if management is lying, someone needs to say so.

He''s been called a hero and a villain for the same trade.

Bill Ackman

Ackman runs highly concentrated books and uses leverage. That's the opposite of conservative.

He'll put 20–30% of the fund in a single position if he believes in it. He also uses options and credit derivatives — his March 2020 COVID hedge was built using credit default swaps, instruments most retail investors have never heard of.

He famously described his risk approach as: "I only invest when the downside is zero and the upside is unlimited" — which sounds great until you lose $4 billion on one trade. The honest version is: he's a high-conviction investor with a high tolerance for pain on the way to being right.

David Einhorn

Einhorn runs a concentrated long/short book with meaningful leverage. He''s comfortable holding short positions for years and absorbing paper losses while waiting for a thesis to play out.

He has described his approach as being comfortable with uncertainty as long as the fundamental analysis is sound. The problem is that being fundamentally right and being market-right are two different things, and the gap between them can be expensive.

Bill Ackman

Ackman has a taste for the finer things and doesn't pretend otherwise. He owns a large Manhattan apartment, he's been photographed at high-end charity events, and his social circle overlaps with New York media, finance, and political elite.

He and his second wife, Neri Oxman, have a high public profile — she's a former MIT professor and design pioneer. He gave $25 million to Harvard (his alma mater), though the relationship became famously strained in 2023 when he led a very public campaign against Harvard's president over campus antisemitism, ultimately contributing to her resignation.

He is not a guy who stays quiet about anything.

David Einhorn

Einhorn is known for being relatively frugal for a hedge fund manager of his stature. He''s a serious poker player, not a yacht person.

He has donated millions to various causes, including a significant amount through his winning poker tournament proceeds. He lives in New York.

He''s notably down-to-earth compared to the hedge fund world''s flashier personalities — no famous cars, no tabloid drama.

Bill Ackman

The COVID hedge in March 2020 is the one. As markets started selling off in late February, Ackman quietly spent $27 million buying credit default swaps — basically insurance on corporate bonds defaulting if the economy collapsed.

He then went on CNBC on March 18, 2020, visibly emotional, and said "hell is coming." The market kept dropping. Three weeks later, he unwound the trade.

The $27 million had turned into $2.6 billion. That's roughly a 100x return in under a month.

He used the proceeds to buy stocks at the market bottom. He then made another fortune as markets recovered.

The whole sequence — hedge, cry on TV, buy the dip, profit — is one of the more remarkable individual trade sequences in recent hedge fund history.

David Einhorn

The Lehman Brothers short is the definitive win. Einhorn began shorting Lehman''s stock in early 2008.

He presented his thesis publicly at a Ira Sohn Conference in May 2008, citing specific accounting concerns about how the firm was valuing its real estate exposure. Lehman''s stock fell immediately.

The firm''s CEO famously called Einhorn a "short and distort" operator. On September 15, 2008, Lehman Brothers filed for bankruptcy.

Greenlight made over $1 billion on the trade. It is one of the most vindicated public short positions in financial history.

Bill Ackman

The Valeant Pharmaceuticals disaster is the one. Ackman built a massive position in Valeant starting in 2015, eventually owning about $4 billion worth of shares.

His thesis was that Valeant's model — aggressively raising drug prices and cutting R&D — was brilliant. Congress, journalists, and eventually the SEC had a different view.

The stock collapsed from $260 to under $10. Ackman spent months publicly defending the position, appearing on CNBC repeatedly to explain why it would recover.

It didn't. He finally sold in 2017 at a loss of approximately $4 billion.

It's the most expensive public loss in hedge fund activism history. The lesson he's cited: don't get emotionally attached to a position, and be faster to recognize when the fundamental thesis has broken.

David Einhorn

The post-2015 period is where the story gets complicated. Greenlight significantly underperformed from 2015 onward.

The fund lost money in 2015, 2018, and 2020. Part of this was a structural problem: value investing broadly underperformed during the growth stock boom.

Part of it was specific bets that went wrong. His short of Tesla was particularly costly — Tesla''s stock rose dramatically while Einhorn remained convinced the company was overvalued, costing the fund hundreds of millions.

He compared Tesla to the fraud companies he''d exposed previously. Tesla kept going up anyway.

The difference between "fundamentally wrong" and "stock going up" is an expensive distinction to make.

Bill Ackman

Bill Ackman grew up in Chappaqua, New York, the son of a real estate finance chairman. He was a history major at Harvard — class of 1988 — then went straight to Harvard Business School.

His first venture was Gotham Partners, a real estate and value investing fund he started in 1992 with a Harvard classmate. It was a disaster.

The fund made concentrated bets on illiquid real estate and had to be wound down by 2003 under serious investor pressure and SEC scrutiny.

He didn't quit. In 2004 he launched Pershing Square Capital Management, this time with a clearer focus: take large stakes in companies, go public with his thesis, and use activist pressure to force management changes.

The approach worked. His reputation was built on detailed, public investment presentations — sometimes running 100+ slides — that became must-reads on Wall Street.

He turned activist investing into something that felt more like journalism than finance: research a company, find what's broken, publish everything, and bet heavily on being right.

David Einhorn

David Einhorn grew up in Demarest, New Jersey, in a family of academics and professionals. He attended Cornell University, graduating summa cum laude with a degree in government.

After a brief stint at Donaldson, Lufkin & Jenrette, an investment bank, he started Greenlight Capital in 1996 at age 27 with $900,000 raised from family and friends.

The fund performed well from the start. Einhorn built a reputation for meticulous fundamental research — reading SEC filings obsessively, questioning management in earnings calls, and occasionally going public with short theses that embarrassed companies.

His 2002 short of Allied Capital, a business development company he argued was mismarking its loan book, started a years-long battle with the company and eventually the SEC. He wrote a book about it.

He was ultimately right.

Bill Ackman

Pershing Square Capital Management is his flagship hedge fund, managing around $16 billion. His most famous public positions have included Canadian Pacific Railway, where he pushed successfully for a new CEO and a turnaround that made Pershing Square hundreds of millions.

He held a massive position in Valeant Pharmaceuticals from 2015 to 2017 — which will come up again. He took a huge bet on General Growth Properties during the 2008 financial crisis when no one else would touch it.

That one returned over $1 billion.

He has also done business on the other side: Pershing Square Holdings is his publicly listed vehicle on Euronext Amsterdam, which lets retail investors access his fund — unusual for a hedge fund of this size. He's been a major Burger King and Restaurant Brands International investor, and he backed Fannie Mae and Freddie Mac preferred shares in a long-running legal battle with the government.

David Einhorn

Greenlight Capital is his hedge fund, managing approximately $1.5 billion today — down significantly from its peak of over $12 billion before a decade of underperformance. Greenlight runs a long/short equity strategy, meaning it simultaneously bets on stocks going up and stocks going down.

The short side is where Einhorn has built his public reputation.

He is also deeply involved in poker. He has finished in the money at the World Series of Poker multiple times, made the final table of the Main Event in 2012, and has won over $4 million in tournament poker.

He donated his 2006 World Series earnings to charity. He has said publicly that poker and investing require the same skills: reading information, managing uncertainty, and knowing when to fold.

Bill Ackman

Harvard College, BA in History, 1988. Harvard Business School, MBA, 1992.

He's been a major Harvard donor — and major Harvard critic — throughout his career. The irony of his most public fight being with his own alma mater was not lost on anyone.

David Einhorn

Cornell University, BA in Government, summa cum laude, 1991. He was a member of the Telluride Association, an honors society that placed particularly sharp students in a semi-independent living and learning program.

He has described his Cornell education as teaching him how to think, not what to think — which is, coincidentally, what good investing requires.