This chapter examines the technologies associated with the conversion of waste heat to power. This chapter starts out outlining different types of waste heat, their sources and usefulness and factors affecting waste heat recovery. This chapter points out that amount of waste heat produced in the world is enormous and should be captured. The largest amount of waste heat is at low temperatures and this can be converted to power using organic Rankine cycle and Kalina cycle. This chapter describes the use of these cycles in some details. The application of ORC for different types of waste heat is specifically emphasized. This chapter then mainly focuses on the use of thermoelectricity to convert waste heat to power. Basic principles of thermoelectricity and the developments of conventional and new thermoelectric materials are briefly described. The role of nanostructured materials to improve the performance of thermoelectric generator is described in detail. Thin film technology, development of different types of TEG modules and applications of TEG for waste heat recovery from the human body, industry and homes, transport systems, regenerative computing, solar waste heat and waste heat from natural gas in remote locations are examined in detail. Finally, the commercialization and future outlook of TEG as well as other methods for direct conversion of waste heat to power are briefly analyzed.