This text deals with the tax treatment of corporate losses, constituting real economic losses in companies, both domestic and cross-border. Firstly, the tax treatment of losses of a company organized as a single entity is examined. This includes time restrictions concerning the access to carry-forward and carry-back losses. Other restrictions, such as basket limitations, restrictions concerning changes in the ownership of tax-loss companies and anti-avoidance provisions are discussed. Secondly, the tax treatment of losses within a group is examined. Again, attention is paid both to domestic and foreign losses. Particular consideration is given to an evaluation of the policy reasons of countries for applying the various rules and restrictions on carrying over corporate losses.