In recent years much interest has been focused on the materials that respond to external stimuli such as temperature, pH, light or presence of biochemical compounds. So called stimuli-sensitive ("smart" or "intelligent") structures are of significant scientific and technological importance. They are used to construct devices, also nano- and microdevices for a variety of applications in the delivery of therapeutics, tissue engineering, bioseparation, sensors or actuators and many others. This review focuses on the stimuli-sensitive polymers and their potential application. In the first part the fundamentals of the behavior in solution of temperature and pH sensitive polymer materials are discussed. Basic information on other stimuli responsive polymers (photo- and biochemical sensitive systems, dual sensitive polymers) are also given. The next paper will be devoted to the application of stimuli-sensitive polymer materials in various areas.