This review summarizes key observations and concepts concerning the role of neuronal populations in specification and control of the direction of movement and isometric force. Large populations of neurons in the motor cortex are engaged with reaching movements. This engagement is fairly early, starting approximately 60 msec following target onset. Single cells are directionally broadly tuned, but the neuronal population carries an unambiguous directional signal. The outcome of this population code can be visualized as a vector that points in the direction of the upcoming movement during the reaction time, during an instructed delay period, and during a memorized delay period. Moreover, when a mental transformation is required for the generation of a reaching movement in a different direction from a reference direction, the population vector provides a direct insight into the nature of the cognitive process by which the required transformation is achieved.