When a polymer solution is emulsified in water and freeze dried, structured polymer microparticles form as a result of phase separation within the emulsion droplets. Under appropriate conditions, emulsification/freeze drying results in hollow polymer particles or microcapsules. The factors influencing the formation of hollow particles and microcapsules will be examined and discussed. Experimental and theoretical evidence will be presented that interfacial free energy minimization after phase separation is the primary factor determining particle structure. Emulsification/freeze drying is a simple route to produce microcapsules from a variety of polymers. Water-soluble compounds can be encapsulated inside the hollow polymer particles. When formed from biodegradable polymers, the microcapsules are attractive for controlled release of the encapsulated material.