Active vision systems can be considered as systems that integrate visual sensing and action. Sensing includes detection of actions/events and results also in specific actions/manipulations. This paper mainly addresses the basic issues in the design of a head-eye system for the study of active-purposive vision. The design complexity of such a head is defined by the activeness of the visual system. Although we have not had the motivation to exactly reproduce the biological solutions in a robot, we claim that the designer should carefully consider the solutions offered by evolution. The flexibility of the behavioral pattern of the system is constrained by the mechanical structure and the computational architecture used in the control system of the head. The purpose of the paper is to describe the mechanical structure as well as the computational architecture of the KTH-head from this perspective.