The theory of ferromagnetic resonance (FMR) and spin wave resonance (SWR) is presented for the general case of multiphase ferromagnets. This could be a magnetic multilayer structure or a material with mixed phases of distinct ferromagnetic materials. We discuss the application of the theory to various systems, with a description of amorphous and nanocrystalline materials which have not received much attention with respect to FMR and SWR. In this respect, we treat these materials in a multiphase manner for the first time, where previous FMR measurements on these types of material have been analysed and interpreted as single phase ferromagnets. Although the general theory is applicable to an N phase material, we show the detailed analysis for two phase systems, giving examples of both magnetic multilayers and mixed double phase ferromagnets of the type for mixed nanocrystalline and amorphous magnetic systems. The theory would also be applicable to systems where a magnetic phase is surrounded by a non-magnetic phase, such as co-deposited systems. We also consider, for the first time, the possibility of the existence of standing spin wave modes in mixed phase and granular materials. The adaptability of the general theory to any form of magnetocrystalline anisotropy and interphase interaction is also discussed.