This textbook is for a one semester introductory course in thermodynamics, primarily for use in a mechanical or aerospace engineering program, although it could also be used in an engineering science curriculum. The book contains a section on the geometry of curves and surfaces, in order to review those parts of calculus that are needed in thermodynamics for interpolation and in discussing thermodynamic equations of state of simple substances. It presents the First Law of Thermodynamics as an equation for the time rate of change of system energy, the same way that Newton’s Law of Motion, an equation for the time rate of change of system momentum, is presented in Dynamics. Moreover, this emphasis illustrates the importance of the equation to the study of heat transfer and fluid mechanics. New thermodynamic properties, such as internal energy and entropy, are introduced with a motivating discussion rather than by abstract postulation, and connection is made with kinetic theory. Thermodynamic properties of the vaporizable liquids needed for the solution of practical thermodynamic problems (e.g. water and various refrigerants) are presented in a unique tabular format that is both simple to understand and easy to use. All theoretical discussions throughout the book are accompanied by worked examples illustrating their use in practical devices. These examples of the solution of various kinds of thermodynamic problems are all structured in exactly the same way in order to make, as a result of the repetitions, the solution of new problems easier for students to follow, and ultimately, to produce themselves. Many additional problems are provided, half of them with answers, for students to do on their own.