This article analyzes the behavior of business entities in the investment process in the economic theory of Dj. M. Keynes. The article examines the role of predel efficiency, interest rates and uncertainty as key factors in investment decisions. It also explains how economic activity can increase as a result of initial investment through the Keynesian multiplier model. With the help of modern economic research and criticism, the Keynesian theory will be reviewed more broadly and its adaptation to new conditions will be analyzed.