This text is about two closely related subjects: thermodynamics and statistical mechanics. Thermodynamics is a general theory of macroscopic systems which provides limits on allowable physical processes involving energy transformations and relations between various measurable quantities. Its power is in its generality. Its limitation is that all quantitative predictions require empirical inputs. Statistical mechanics provides a microscopic foundation for thermodynamics and can be used to make quantitative predictions about macroscopic systems. Thermodynamics has always been important, but is of particular relevance at present because of important policy and engineering issues that require an understanding of thermal systems. These issues include global climate change, the search for new sources of energy, and the need for more efficient uses of energy. Statistical mechanics has become much more important in physics and related areas because its tools can now be implemented on computers and are much more widely applicable. These applications include lattice gauge theories in high energy physics, many problems in astrophysics, biological physics and geophysics, as well as topics traditionally considered outside of physics such as social networks and finance.