The author deals with conditional symmetry of partial differential equations --- roughly speaking with the symmetry of certain subset of solutions. Such a symmetry can be used in generalization of invariance principle, reduction and linearization of equations and in the process of obtaining solutions, which cannot be obtained by some classical methods. Conditional symmetry and solutions are investigated for equations of heat transfer, accoustics, Schrödinger, Boussinesq, Korteweg-de Vries, Maxwell, d'Alembert and Dirac.