Employment presupposes an activity exercised under the direction of a person. In the presence of a causal link with the payment of any remuneration in cash or kind, employment constitutes the source of income and, taking into account this umbrella function, its specific rules should generally apply for taxing purposes in purely domestic and cross-border situations. While the personal tax nexus follows the ordinary rules, the objective nexus of income from employment mostly relates to the actual place of exercise of activity. Some adaptations may apply, also in the framework of specific treaty rules, when the place of activity is missing, or hard to determine. Most cross-border employment income generates full exposure to double taxation, except in the case of a weak objective nexus with States other than that of residence of the employee.