This review focuses on the thermochromic phenomena in polymers. The content of the review is structured according to the thermochromic material type. Chapter 1 will give an overview about polymers exhibiting thermochromic properties themselves. Thermochromic polymer systems obtained by doping the polymer matrix with thermochromic additives are reported in Chapter 2 and Chapter 3 reviews on polymer systems in which the interaction between the polymer matrix and an incorporated additive is the origin of thermochromism. Finally, in the fourth chapter polymer systems changing their transparency with temperature by switching reversible between a transparent and a light scattering state are reviewed.