The description of ‘smart’ is applied to a variety of systems that are capable of reacting to a change in environment and providing a unique response. The scope of this review is focused on environmentally responsive smart particles, with a secondary emphasis on in-situ sensor particles for in-process characterization. This review provides information on particles developed to ‘smartly’ respond to mechanical, thermal, chemical, biological, electromagnetic, magnetic, or electrical stimuli. The review covers multiple aspects of “smart” particles, including their applications, mode of action, preparation methods, and fate. Smart particles used for commercial and experimental applications are included.