This chapter addresses the two most common ways of using the Lie symmetries of partial differential equations, namely, the determination of particular exact solutions that are invariant with respect to a Lie group of point transformations, and the generation of new solutions starting from existing ones. The first strategy uses the infinitesimal version of the transformations, whereas the second one the finite expression of groups of transformations. The search for invariant solutions is the widely used application of Lie group theory. In some cases, invariant solutions describe meaningful physical situations (for instance, self-similar solutions or traveling wave solutions). Nevertheless, it has to be remarked that, in general, not always the admitted symmetries of the partial differential equations leave physically relevant initial and/or boundary conditions invariant, thus preventing us from solving satisfactorily such problems. Additive or multiplicative separable solutions that can be obtained as invariant solutions, and invariant solutions with respect to an r-parameter subgroup of the symmetries are discussed. Finally, the problem of classifying inequivalent invariant solutions using optimal systems of Lie subalgebras is described.