Preliminaries.- Perturbations of Linear Systems.- Linear Systems with Slowly Varying Coefficients.- Linear Dissipative and Piecewise Constant Systems.- Nonlinear Systems with Autonomous Linear Parts.- The Aizerman Problem.- Nonlinear Systems with Time-Variant Linear Parts.- Essentially Nonlinear Systems.- The Lur'e Type Systems.- The Aizerman Type Problem for Nonautonomous Systems.- Input - State Stability.- Orbital Stability and Forced Oscillations.- Positive and Nontrivial Steady States.