Symmetry has a strong impact on the number and shape ofsolutions to variational problems. This has been observed,for instance, in the search for periodic solutions ofHamiltonian systems or of the nonlinear wave equation; whenone is interested in elliptic equations on symmetric domainsor in the corresponding semiflows; and when one is lookingfor "special" solutions of these problems.This book is concerned with Lusternik-Schnirelmann theoryand Morse-Conley theory for group invariant functionals.These topological methods are developed in detail with newcalculations of the equivariant Lusternik-Schnirelmanncategory and versions of the Borsuk-Ulam theorem for verygeneral classes of symmetry groups. The Morse-Conley theoryis applied to bifurcation problems, in particular to thebifurcation of steady states and hetero-clinic orbits ofO(3)-symmetric flows; and to the existence of periodicsolutions nearequilibria of symmetric Hamiltonian systems.Some familiarity with the usualminimax theory and basicalgebraic topology is assumed.