Hydrogels are capable of adsorbing large quantities of water due to their hydrophilic three-dimensional polymeric networks; thus, their physical properties are similar to soft tissues such as cartilage and muscle. Stimuli-responsive smart hydrogels are particularly interesting because of their ability to respond to various exogenous and/or endogenous stimuli such as pH, thermal, light, magnetism, etc. Because of their versatile and unique properties, smart hydrogels show great potential in controlled drug release, tissue engineering scaffolds, solid/water stabilization, etc., wherein a rapid responsive property and outstanding mechanical properties are highly desired. The degree of responsiveness and mechanical properties of hydrogels are highly dependent on their polymeric network structures. Thus, the network structure design strategies are of great interest and particular importance for various applications. In this paper, we review various network design strategies to build smart hydrogel networks with rapid responsiveness and/or high mechanical properties, as well as the emerging applications of smart hydrogels such as controlled release systems, detection sensors, soft valves, and responsive actuators. The perspectives as well as current challenges facing the applications of such innovative stimuli-responsive polymeric materials are also addressed.