This thesis addresses strictly positive real systems and their application in the control of uncertain dynamical systems, as for exemplare, in adaptative control with memory and in variable structure systems with sliding motions. The results of this work extend the knowledge about strictly positive real systems and their application in control systems, and offer new control laws that improve the existing configurations, as for example in transient estimation, state variable access and practical details of implementation. The model reference adaptive control, the stabilization of uncertain dynamical systems and the servo system problem are considerad new results are obtained. For the strictly positive real systems, equivalent definitions, conditions to turn a transfer matrix into a strictly positive real and relations of the variable structure systems with sliding motions and high gain system with the strictly positive real systems are presented. Some of the results above are extended for the linear time varying sytems.