Spontaneous emission control for rare earth ions implanted inside planar multidielectric microcavities is investigated theoretically and experimentally. Rigorous classical electrodynamics analyses are presented and compared in order to compute the electromagnetic power provided by sources located inside planar dielectric multilayer structures. Radiation patterns and lifetime measurements are performed in Ta 2 O 5 /SiO 2 microcavities implanted with erbium and praseodymium ions. Although we demonstrate a significant enhancement of the spontaneous emission in a direction normal to the layer (up to 30%), we show that a large amount of the emitted power is carried by the guided modes of the structures.