Kálmán published several seminal papers during the sixties, which rigorously established what is now known as the state-space representation of dynamical systems. He introduced the formal definition of a system, the notions of controllability and observability, eventually leading to the Kalman decomposition. Kálmán also gave groundbreaking contributions to the theory of optimal control and provided, in his joint work with J. E. Bertram, a comprehensive and insightful exposure of stability theory for dynamical systems. He also worked with B. L. Ho on the minimal realization problem, providing the well known Ho-Kalman algorithm.