Over about the last two decades, many kinds of stimuli-responsive polymer gels in response to the change in their surroundings such as solvent composition, temperature, pH, and supply of electric field, etc., have been developed. They have attracted much attention as intelligent (or smart, biomimetic) materials which have sensor, processor and actuator functions. Applications to actuator (artificial muscle), biosensor, drug delivery systems, purification or separation systems, etc. are extensively studied. In this review, design of stimuli-responsive gels and their application to bio- or biomimetic materials will be discussed as well as our recent studies on novel self-oscillating gels.