Recently, "smart" hydrogels with either shape memory behavior or reversible actuation have received particular attention and have been further developed into sensors, actuators, or artificial muscles. These three-dimensional polymer networks with the capability of shape deformations show a volume phase transition after being triggered by external physicochemical stimuli. Here, we review the recent advancements and the different types of shape memory hydrogels (SMHs). In addition, stimuli-responsive hydrogel actuators have been investigated with a special focus on their stimulation, their motion-deformation strategies and on the fabrication technologies adopted in hydrogel-based actuators, and finally their applications are described and discussed using specific examples.