Time-Temperature Indicators (TTIs) are small devices which provide easy-to-read, visual information regarding the thermal history of perishable products. They are typically designed as small labels suitable for application onto the packaging of single product items. TTI labels are inexpensive, especially when compared to temperature electronic recorders, which are intended to monitor warehouses or means of transportation, rather than monitor single product items. Current and potential fields of application comprise food, pharmaceutical, biomedical and floral products. In the present paper the functioning principles of several TTIs are analysed with regard both to theoretical and practical aspects. There are various basic principles involved in the design of TTIs ranging from chemical reactions to physical phenomena. These principles include solid phase chemical reactions, biochemical reactions, biological systems, the transition of colour indicators in reacting phases, molecular diffusion, motion of viscous liquids, melting or glass transition of solids, gas permeation, osmosis, and nanoparticles. A selection of the most relevant patents was made according to the following criteria: the commercial success of the TTIs or their potential commercial viability, the recentness of the publications, and the originality of the fundamental principles of the inventions.