The article explores the concept of involuntary unemployment in Keynesian theory and its evolution over time. It discusses how Keynes initially addressed this issue in his General Theory, the challenges faced, and argues that involuntary unemployment is not fully captured in traditional models. The article also examines different interpretations of involuntary unemployment by various economists and evaluates the criticisms posed by anti-Keynesian thinkers like Friedman and Lucas. Finally, it delves into the role of involuntary unemployment in newer Keynesian models.