IEC 60601-2-1:2009 applies to the basic safety and essential performance of electron accelerators in the range 1 MeV to 50 MeV, used for treatment of patients. This particular standard establishes requirements to be complied with by manufacturers in the design and construction of electron accelerators for use in radiotherapy; it does not attempt to define their optimum performance requirements. Its purpose is to identify those features of design that are regarded, at the present time, as essential for the safe operation of such medical electrical equipment. It places limits on the degradation of medical electrical equipment performance beyond which it can be presumed that a fault condition exists and where an interlock then operates to prevent continued operation of the me equipment. This third edition cancels and replaces the second edition published in 1998 and its Amendment 1 (2002). It constitutes a technical revision. This third edition addresses the following issues not covered in previous editions: – alignment with the new relevant collateral standards; – new technologies in radiotherapy, including stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT), intensity modulated radiotherapy (IMRT), electronic imaging devices (e.g. EPID) and moving beam radiotherapy (dynamic therapy).