This chapter focuses on imaging protocols to discuss correction strategies and the principles that lead to their formulation. It discusses other aspects such as tolerances used and action levels, which influence the correction of imaging errors; how these affect planning and treatment margins; and why these issues need to be addressed before implementing a correction strategy. In imaging terms, tolerance is the allowable discrepancy between the planned treatment and the treatment delivered. A tolerance is required for most aspects of radiation treatment that involve patient and staff interaction. Due to the differences in stability, different tolerance levels are required for on-treatment imaging when patients are treated using different immobilisation equipment. The treatment aim will have an effect on imaging tolerances. Altering the treatment planning margins in such a way will affect the imaging tolerance for that plan. Some specialised treatments may use a very conformal Planning Target Volume margin.