This paper analyzes major patterns and trends in entrepreneurship among technology-based university alumni since the 1930s by asking two related research questions: (1) Who enters entrepreneurship, and has this changed over time? (2) How does the rate of entrepreneurship vary with changes in the entrepreneurial business environment? We describe findings based on data from two linked datasets joining Massachusetts Institute of Technology (MIT) alumni and founder information. New company formation rates by MIT alumni have grown dramatically over seven decades, and the median age of first time entrepreneurs has gradually declined from about age 40 (1950s) to about age 30 (1990s). Women alumnae lag their male counterparts in the rate at which they become entrepreneurs, and alumni who are not U.S. citizens enter entrepreneurship at different (usually higher) rates relative to their American classmates. New venture foundings over time are correlated with measures of the changing external entrepreneurial and business environment, suggesting that future research in this domain may wish to more carefully examine such factors.