The purpose of this document is to educate TMDL practitioners on Microbial Source Tracking (MST) and how it can be used to support TMDL development and implementation. The document covers: 1) Potential uses of MST 2) Brief descriptions of common MST methods 3) Factors for selecting an MST method and designing an MST study 4) Examples of MST studies used to support TMDL development or implementation MST can support bacteria TMDL development and implementation during the assessment process, the allocation analysis, and/or development of an implementation plan. Specifically, MST can help to identify the sources that contribute to the observed impairment, determine which sources are likely the most dominant in the watershed of interest, and focus management actions through targeting implementation and identifying controls that are appropriate and relevant to the identified sources.