A country's gross external debt (or foreign debt) is the liabilities that are owed to nonresidents by residents. The debtors can be governments, corporations or citizens. External debt may be denominated in domestic or foreign currency. It includes amounts owed to private commercial banks, foreign governments, or international financial institutions such as the International Monetary Fund (IMF) and the World Bank.

External debt measures an economy's obligations to make future payments and, therefore, is an indicator of a country's vulnerability to solvency and liquidity problems. Another useful indicator is the net external debt position, which equals gross external debt minus external assets in the form of debt instruments. A related concept is the net international investment position (net IIP). Provided that debt securities are measured at market value, the net external debt position equals the net IIP excluding equity and investment fund shares, financial derivatives, and employee stock options.

According to the International Monetary Fund's External Debt Statistics: Guide for Compilers and Users, "Gross external debt, at any given time, is the outstanding amount of those actual current, and not contingent, liabilities that require payment(s) of principal and/or interest by the debtor at some point(s) in the future and that are owed to nonresidents by residents of an economy."

In this definition, the IMF defines the key elements as follows:

Generally, external debt is classified into four heads:

However, the exact treatment varies from country to country. For example, while Egypt maintains this four-head classification, in India it is classified in seven heads:

Sustainable debt is the level of debt which allows a debtor country to meet its current and future debt service obligations in full, without recourse to further debt relief or rescheduling, avoiding accumulation of arrears, while allowing an acceptable level of economic growth.

External-debt-sustainability analysis is generally conducted in the context of medium-term scenarios. These scenarios are numerical evaluations that take account of expectations of the behavior of economic variables and other factors to determine the conditions under which debt and other indicators would stabilize at reasonable levels, the major risks to the economy, and the need and scope for policy adjustment. In these analyses, macroeconomic uncertainties, such as the outlook for the current account, and policy uncertainties, such as for fiscal policy, tend to dominate the medium-term outlook.