Light emitting diodes find increasing usage in various applications household, commercial, signage, automotive, and backlighting to name a few. A key feature of LEDs is that all waste heat needs to be conducted away from the package in order to manage the junction temperature, and thus performance and reliability. Thermal design of device and board level packages will determine not only the long-term reliability of LEDs, but also affect the stability of light output and color response. In this presentation we will scrutinize the thermal resistances to heat transported from the LED junction to the ambient environment. The presentation will identify thermal challenges associated with LED packaging from die/LED package level, to circuit/board level. We will review how properties of MCPCB substrates and thermal interface materials can affect heat transport and junction temperature. Both end-of-line performance and long term reliability will be addressed. In addition we will propose working examples of thermally conductive materials solutions, and/or models and address processing and manufacturability.