While the perception of heat and cold is ancient, precise measurement began in the sixteenth century. The invention of the steam engine in the seventeenth century and its subsequent refinements are discussed. Competing theories on the nature of heat are presented, along with advancements in thermometers and temperature scales. After this the scientists were equipped for studying various properties of heat. By the late eighteenth century, concepts such as latent and specific heats were introduced and their experimental measurements were performed. The three modes of heat transfer—conduction, convection, and radiation—are explained, along with the associated laws. The kinetic theory of gases is explored through Boyle’s law, Charles’s law, Gay-Lussac’s law, and the ideal gas equation. The chapter concludes with an introduction to entropy and the description of thermodynamic principles, highlighting contributions by Helmholtz and Gibbs.