From the outset, expectations were a central part of the first macrodynamic models and early growth theories. In the 1940s, a third line of research emerged which questioned the capacity of an economy to reach full-employment equilibrium. Starting with Alvin Hansen (1938) and culminating with Oskar Lange (1944), the crux of the debate evolved from the existence of full employment equilibrium to analysis of its stability, suggesting an increased role for expectations and finally challenging the economic system’s global stability. The present paper traces those debates through the contributions of Hansen, Paul Samuelson and Lange. Using archives materials, we show that while Samuelson’s analysis of instability remained implicit, his correspondence reveals that he encouraged Oskar Lange to examine it more carefully. Lange’s results are presented in his 1944 Cowles Commission Monograph. We point out that his contribution cannot be understood in isolation either from his exchanges with Samuelson or the way that Keynesian ideas were being interpreted in the United States. The paper finally questions Samuelson’s view on instability and expectations.