In 1933, he moves to Amsterdam to study mathematical economics under Jan Tinbergen. Koopmans extends his explorations to econometrics and statistics. His supervisor moves to Leiden so the PhD degree is granted by the University of Leiden in 1936. The title of the thesis is "Linear regression analysis of economic time series".

In 1955 he accepts an appointment at Yale. In that period, his research is chiefly on optimum allocation over time. But there is also another six-year stint as Director (1961-1967). 

For the two academic years 1936-1938, Koopmans takes over Tinbergen’s lectures at the School of Economics in Rotterdam.

He attends the University of Utrecht at age 17. In the first three years, his emphasis is on mathematics. In 1930, he switches to theoretical physics. The early thirties bring the “great depression”; Marxist economists describe it as the great crisis of capitalism. Karl Marx's “Das Kapital” is the first book in economics that he studies. 

Tjalling Koopmans shares the 1975 Nobel Prize in Economics with Leonid Kantorovich "for their contributions to the theory of optimum allocation of resources". Koopmans has renewed, generalized, and developed methods for analysis of the classical problem of economics as regards the optimum allocation of scarce resources.

He spends two years in Geneva, working on the construction of a model of business cycles for United Kingdom.

He spends the year 1968-1969 at the Center for Advanced Study in the Behavioral Sciences at Stanford, California.

When the war spreads to Western Europe, he manages to move with his wife and their six-week old daughter to the United States in 1940. He works at Princeton University (1940/41) and the British Merchant Shipping Mission in Washington, D.C.

Then he moves to Chicago where he joins the Cowles Commission for Research in Economics, affiliated with the University of Chicago. In 1948, he becomes Director of Research for a six-year period.

He marries Truus Wanningen, whom he had met in Amsterdam.

Tjalling Koopmans is born on in Graveland, in the Netherlands.

Tjalling Koopmans dies in New Haven.