Despite its proven accuracy and the potential for improved dose distributions to influence treatment outcomes, the long calculation times previously associated with MC simulation rendered this method impractical for routine clinical treatment planning. However, the development of faster codes optimized for radiotherapy calculations and improvements in computer processor technology have substantially reduced calculation times to, in some instances, within minutes on a single processor. Several commercial vendors have released or are currently in the process of releasing MC algorithms for photon and/or electron beam treatment planning. Consequently, the accessibility and use of MC treatment planning algorithms may well become widespread in the radiotherapy community. With MC simulation, dose is computed stochastically using first principles; this method is therefore quite different from conventional dose algorithms. Issues such as statistical uncertainties, the use of variance reduction techniques, the ability to account for geometric details in the accelerator treatment head simulation, and other features, are all unique components of a MC treatment planning algorithm. Successful implementation by the clinical physicist of such a system will require an understanding of the basic principles of MC techniques. Educational Objectives: 1. To provide an educational review of the physics of the MC including discussion of methods used for coupled photon and electron transport. 2. To review the methods used to improve the simulation efficiency. 3. To briefly review the vendor transport codes currently used for clinical treatment planning. 4. To describe the development of treatment head models for clinical treatment planning. 5. To discuss the factors associated with MC dose calculation within the patient‐specific geometry, such as statistical uncertainties, CT‐to‐material, and reporting of dose‐to‐medium versus dose‐to‐water. 6. To discuss the issues associated with experimental verification of MC algorithms. 7. To briefly review the potential clinical implications of MC calculated dose distributions. 8. To provide example timing comparisons of the major vendor MC codes in the clinical setting.