We examine the determinants of private equity returns using a newly constructed database of 7,500 investments worldwide over forty years. The median investment IRR (PME) is 21% (1.3), gross of fees. One in ten investments goes bankrupt, whereas one in four has an IRR above 50%. Only one in eight investments is held for less than 2 years, but such investments have the highest returns. The scale of private equity firms is a significant driver of returns: investments held at times of a high number of simultaneous investments underperform substantially. The median IRR is 36% in the lowest scale decile and 16% in the highest. Results survive robustness tests. Diseconomies of scale are linked to firm structure: independent firms, less hierarchical firms, and those with managers of similar professional backgrounds exhibit smaller diseconomies of scale.