Temperature is one of the most important factors affecting the state and behavior of materials. In situ heating transmission electron microscopy (TEM) is a powerful tool for understanding such temperature effects, and recently in situ heating TEM has made significant progress in terms of temperature available and resolution attained. This article briefly describes newly developed specimen-heating holders, which are useful in carrying out in situ heating TEM experiments. It then focuses on three main applications of these specimen holders: solid–solid reactions, solid–liquid reactions (including highresolution observation of a solid–liquid interface, size dependence of the melting temperatures of one-, two- and three-dimensionally reduced systems, size dependence of the contact angle of fine metal liquid, and wetting of Si with liquid Au or Al) and solid–gas reactions. These results illustrate the benefit of in situ heating TEM for providing fundamental information on temperature effects on materials.