Mixed-valent systems based on Ce, Sm, Eu and Yb exhibit a wide range of behaviour with respect to valence changes under the application of pressure. The authors present a semi-phenomenological model for this behaviour based on competition effects between the usual elastic energy cost and the magnetic energy gain due to valence fluctuations. For the latter they use a mean-field Anderson lattice description and incorporate the effects of pressure by introducing a volume dependence to the Anderson model parameters in _f and Delta . In contrast to existing models such as the Kondo Volume Collapse theory of Allen and Martin (1982), which describes magnetic to non-magnetic transitions without sizable valence change (e.g. gamma -Ce to alpha -Ce), the Anderson lattice model developed here describes systems with both small and large valence changes: the transition can be continuous (e.g. EuPd₂Si₂) or discontinuous (EuPd₂Si₂ alloyed with Au).