Thermal management is one of the most essential issues for LED applications. The output power, efficiency, emission spectrum and reliability of the LED chip and phosphor are functions of temperature. Without proper thermal management, the output of the LED could depart from the desired performance. Basic heat transfer phenomena and corresponding calculations are introduced as well as the thermal behaviors and concerns of the essential components of an LED. A quick method to estimate the thermal resistance of an LED package is provided based on the shape factor of the geometry at the end of this chapter.