Lawrence R. Klein was one of the most important figures in the collective development of macroeconometric modeling, a novel scientific practice that dominated macroeconomics throughout the first decades of the second half of the twentieth century. Understanding how Klein developed his identity as a macroeconometrician and how he forged a new scientific practice of macroeconometric modeling during 1938–55 is essential for drawing a clear picture of Klein’s importance in the creation and further development of macroeconometric modeling. Toward this aim, I focus on Klein’s early trajectory as a student of economics and an economist, and particularly examine the extent to which the people and institutions that Klein encountered helped him shape his own image of economics, his identity as an economist, and a new scientific practice in the United States. I describe Klein’s contribution as a new way of producing scientific knowledge that consisted in the construction and use of complex tools (macro-econometric models) within specific institutional configurations (econometric laboratories) used for explicit policy and scientific objectives in which the well-defined roles of experts (arranged in scientific teams) were embodied within a new scientific practice (macroeconometric modeling).