Activate provides an environment for modeling systems where continuous-time dynamics and discrete-time and event driven dynamics interact. Strictly speaking such systems are hybrid systems. But a hybrid system is commonly seen as a set of dynamical systems described by differential equations associated each with a mode. The systems associated with different modes may share some of their variables and/or states. At any time only one mode is active; the equations associated with other dynamical systems are not present. The passage from one mode to another can be conditioned on the values of variables and states of the active mode. The events causing mode changes can re-initialize system variables specifying the initial condition of the dynamical system associated with the new mode.