Aid (also known as international aid, overseas aid, or foreign aid, especially in the United States) is a voluntary transfer of resources from one country to another, given at least partly with the objective of benefiting the recipient country. It may have other functions as well: it may be given as a signal of diplomatic approval, or to strengthen a military ally, to reward a government for behaviour desired by the donor, to extend the donor's cultural influence, to provide infrastructure needed by the donor for resource extraction from the recipient country, or to gain other kinds of commercial access. Humanitarianism and altruism are, nevertheless, significant motivations for the giving of aid. Aid may be given by individuals, private organisations, or governments. Standards delimiting exactly the kinds of transfers that count as aid vary. For example, aid figures may or may not include transfers for military use: to cite one instance, the United States included military assistance in its aid figure until 1957 but no longer does. Another issue is whether to count remittances by expatriate workers to family members in their home countries as aid. This constitutes a large but difficult to measure international flow of funds.

Depending on the definition, loans may or may not be counted as aid.

Even if the principles of a definition are set, it remains difficult to determine the effective flow of aid because aid is fungible: receiving aid may free up funds in the recipient country for use in non-aid projects that could not have been undertaken had the aid not been received. For example, receiving food aid may enable a government to divert funds from its own food-support budget to its military budget. In that case the net effect of the aid is military although the aid money might actually be spent on food.

Official organisations and those scholars who are primarily concerned with government policy issues frequently include only government-sourced aid in their aid figures, omitting aid from private sources. The most widely used measure of aid, "Official Development Assistance" (ODA) is such a figure. It is compiled by the Development Assistance Committee of the Organisation for Economic Co-operation and Development. The United Nations, the World Bank, and many scholars use the DAC's ODA figure as their main aid figure because it is easily available and reasonably consistently calculated over time and between countries. The DAC consists of 22 of the wealthiest Western industrialised countries plus the E.U.; it is a forum in which they coordinate their aid policies.

Aid existed in ancient times. More recently, in the nineteenth century, some private aid flowed from the Western countries to the rest of the world; missionary schools are an example. In the nineteenth and early twentieth centuries, aid from governments was tiny compared to present levels, consisting mostly of occasional humanitarian crisis relief. Some transfers that would now be counted as aid, however, came under the purview of colonial office budgets. It was at the end of World War Two, in the contexts of European reconstruction, decolonisation, and cold war rivalry for influence in the third world, that aid became the major activity that it is today.