Shape memory polymers (SMPs) are a group of active polymers that has attracted intense research interest in the past decade. The underlying mechanisms for shape memory and recovery can be classified into two categories: the first one is due to macromolecular chain mobility change during glass transition, and the second one is due to the so- called “phase evolution”. Whilst the first mechanism corresponds to a very specific physical process, the second mechanism includes a large variety of physical processes or phenomenological descriptions of physical processes where their mechanics share some common features. This paper attempts to review these mechanisms, constitutive model framework, and how these frameworks can be applied to specific SMP systems. The SMPs reviewed in this paper include amorphous polymers based SMPs, two-way SMPs due to stretched induced crystallization, shape memory elastomeric composites, and light activated polymers. While SMPs are still one of the forefronts in materials research and new SMPs emerge quickly, the SMPs reviewed in this paper can be representatives and the model frameworks can be applied to many new systems developed.