Dramatic springtime ozone losses have occurred over the Antarctic every year since the early 1980s. During a six-week period between mid-August and the end of September, more than 99% of the ozone is destroyed in a layer 3–5km deep at altitudes between 15 and 21km. With the exception of one year (2002) the ozone loss has remained until at least the end of October and sometimes into December, depending on the stability of the Antarctic vortex in each particular year. In contrast, the ozone loss that occurs over the Arctic varies enormously from year to year. In some years there is little or no loss; in other years as much as 60% or 70% of the ozone is lost at some altitudes. This chapter looks in some detail at the main factors that determine the ozone loss over the two poles using a mixture of observational and modeling studies.