In this thesis we study the harmonicity of smooth maps between Rieman- nian manifolds endowed with some special geometrical structures (Sasakian, Kenmotsu, Kahler, f -structures, generalized Sasakian). The most of maps are generalizations of holomorphic maps, namely it intertwines the geomet- rical structures. We also obtain some results on spectral theory and stability of harmonic maps. We give conditions for a harmonic map to be a har- monic morphism. For most of the results we give some nice applications, for instance in the theory of immersions.