Biomechanics of standing posture: the relation between the center of mass and the center of pressure. The postural control is as complex as the movement control. Two biomechanical variables used to study postural control are the center of mass (COM) and the center of the forces applied in the support, the center of pressure (COP). The aim of this paper is to discuss the relation between these variables. The balance swing is the natural oscillation of the body during standing and is traditionally represented by COM displacement. The COP is related to postural control because it is due to the neuromuscular response to COM displacement. There are time domain and frequency domain differences between COM and COP. To control balance during standing, it is necessary to minimize the difference between COP and COM. The COM can be estimated by three procedures: kinematics, kinetics, and filtering. Acting as important variables for postural studies, COP and COM have different measurement procedures and different physical meaning, thus suggesting also different importance for balance control.