Three-dimensional (3D) printing allows for significant simplifications in the design cycle through the obvious ease with which complex geometries can be manufactured. However, the resulting components have a fixed set of properties that depend on the geometric design and inherent material properties of the printing material. Four-dimensional (4D) printing relaxes this rigidity in geometry and structural characteristics and provides means to alter and tune the resulting response. The tunability in properties can be achieved through various routes. This chapter is focused on 4D printing structures with programmable properties through the use of shape memory polymers. The chapter provides a comprehensive assessment of the challenges associated with the use of this smart material including dimensional stability issues and incomplete local recovery. The complex interactions between material deposition direction and dimensional changes and shape recovery are also highlighted and discussed. I