This chapter describes molecules that selectively react to force when embedded in polymer systems, so-called mechanophores. Such mechanophores are important tools for harnessing and understanding force-induced events within the field of polymer mechanochemistry. Due to the large portfolio of mechanophores and methods for their activation, which have been researched in recent years, many applications have been realized ranging from drug delivery, catalysis, and self-healing, to force-sensing. In this chapter, mechanisms of the force-induced activation of mechanophores are classified, methods for the exertion of force are summarized, and mechanophore applications and their detection using optical properties are described.