Different polymers can be combined into a single material in many ways, which can lead to a wide range of phase behaviors that directly influence the associated physical properties and ultimate applications. Four factors control polymer-polymer phase behavior: choice of monomers, molecular architecture, composition, and molecular size. Current theories and experiments that deal with the equilibrium thermodynamics and non-equilibrium dynamics of polymer mixtures are described in terms of these experimentally accessible parameters. Two representative molecular architectures, binary linear homopolymer mixtures and diblock copolymers, exhibiting macrophase separation and microphase segregation, respectively, are examined in some detail. Although these model systems are fairly well understood, a myriad of mixing scenarios, with both existing and unrealized materials applications, remain unexplored at a fundamental level.