Abstract
Generalized Landau-Ginsburg models for systems with coupled order parameters are introduced. Detailed discussion is given for a particular model with biquadratic coupling between two (second-order) order parameters. This includes the complete, rather interesting, phase diagrams within the Landau-theory approximation and a discussion of fluctuation effects. For these, simple expectations are presented based on the topology of the Landau free-energy surfaces and renormalization group results including an identification of the limitations of the latter. It is argued that these models are very general and relevant to many systems of experimental interest.