Athletes commonly use various forms of real and simulated altitude exposure combined with sea level training to enhance sea-level performance. Altitude can be simulated through use of apartments, tents and masks that allow athletes to breathe air containing less oxygen. There is clear evidence that altitude exposure combined with sea-level training enhances endurance performance especially when the altitude is real. It is unclear what the optimum level of altitude and length of exposure is to enhance performance, although researchers generally adopt 3-4 weeks at moderate altitude (~3000m). There is also little evidence to suggest how long the performance effect persists for. Large gains in VO2max have been reported following actual altitude exposure that may be partly mediated via an increase in red cell volume or haemoglobin mass. However, exposure to simulated altitude appears to have a little effect on VO2max probably due to differences in daily exposure time. The effects of altitude exposure on sprint performance are limited to several investigations that demonstrate a tendency for improved performance, especially in repeated sprints.