We used the (14)C-deoxyglucose method to map the functional activity in the cortex of the lateral and medial parietal convexity, the intraparietal and the parietoccipital sulci of monkeys which either reached and grasped a 3D-object or observed the same reaching-to-grasp movements executed by a human. Execution of reaching-to-grasp induced activations in the superior parietal areas SI-forelimb/convexity, PE, PE caudal (PEc); in the intraparietal areas PE intraparietal (PEip), medial intraparietal (MIP), 5 intraparietal posterior, ventral intraparietal (VIP), anterior intraparietal (AIP), lateral intraparietal dorsal; in the inferior parietal areas PF, PFG, PG; in the parietoccipital areas V6, V6A-dorsal; in the medial cortical areas PGm/7m and retrosplenial cortex. Observation of reaching-to-grasp activated areas SI-forelimb/convexity, PE lateral, PEc, PEip, MIP, VIP, AIP, PF, V6, PGm/7m, 31, and retrosplenial cortex. The common activations were stronger for execution than for observation and the interhemispheric differences were smaller for observation than for execution, contributing to the attribution of action to the correct agent. The extensive overlap of parietal networks activated for action execution and observation supports the "mental simulation theory" which assigns the role of understanding others' actions to the entire distributed neural network responsible for the execution of actions, and not the concept of "mirroring" which reflects the function of a certain class of cells in a couple of cortical areas.