Ferromagnetic Invar and Elinvar alloys are considered as ferromagnets with strong magnetoelastic interactions. A systematic treatment of the phenomenological theory of magnetoelastic interactions in Invars and Elinvars is given. The thermodynamic quantities (thermal expansion, elastic constants, magnetic properties) and dynamical properties (propagation of acoustic waves and spin waves) are calculated theoretically. The combined effect of magnetization fluctuations and volume deformations is considered, as well as the effect of fluctuations on the thermodynamic properties. The theoretical results are compared to experiment and it is shown that the phenomenological theory gives a satisfactory quantitative explanation of the anomalies in the properties of Invars and Elinvars. The cause of the strong magnetoelastic interaction in Invars is discussed.