Inertial manifolds are a modern tool in the qualitative theory of partial differential equations, they contain the global attractor and allow for a reduction to a finite-dimensional system. In this talk we give an introduction and present a geometric approach to inertial manifolds for nonautonomous dynamical systems. We present a new application to time-dependent evolution equations. It is based on the attached paper which will appear in the Journal of Dynamics and Differential Equations.