The requirements to be satisfied by temperature-indicating pigments are stated and the development of pigments to meet this specification is described. The extension to multiple transition paints and the development of binding materials and the composition of the paints are then discussed. Methods of application of the paints and the recording of results are considered. Factors influencing the temperature—colour change relations are then examined; they include ( a) the time effect, ( b) the effect of liquid films and gases, ( c) the effect of heat transfer by conduction and radiation, and ( d) redistribution of temperatures during cooling of the body. Consideration is given to the estimated accuracy of the temperature indicated. A review of uses in (i) Mechanical Engineering, (ii) Electrical Engineering, (iii) Chemical Engineering and Metallurgy, and (iv) Teaching, is made.