Determining the boundaries between the continents is generally a matter of geographical convention and consensus. Several slightly different conventions are in use. The number of continents is most commonly considered seven (in English-speaking countries) but may range as low as four when Afro-Eurasia and the Americas are both considered as single continents. An island can be considered to be associated with a given continent by either lying on the continent's adjacent continental shelf (e.g. Singapore, the British Isles) or being a part of a microcontinent on the same principal tectonic plate (e.g. Madagascar and Seychelles). An island can also be entirely oceanic while still being associated with a continent by geology (e.g. Bermuda, the Australian Indian Ocean Territories) or by common geopolitical convention (e.g. Ascension Island, the South Sandwich Islands). Another example is the grouping into Oceania of the Pacific Islands with Australia and Zealandia.

There are three overland boundaries subject to definition:

While today the isthmus between Asia and Africa is navigable via the Suez Canal, and that between North and South America via the Panama Canal, these artificial channels are not generally accepted as continent-defining boundaries in themselves. The Suez Canal happens to traverse the Isthmus of Suez between the Mediterranean Sea and the Red Sea, dividing Africa and Asia. The continental boundaries are considered to be within the very narrow land connections joining the continents.

The remaining boundaries concern the association of islands and archipelagos with specific continents, notably:

Historically in Greco-Roman geography, "Africa" meant Ancient Libya, and its eastern extent was taken to be around Marmarica, at the Catabathmus Magnus. This was not considered to be a continent. As wider knowledge of geography developed, the shape of the African landmass (and Egypt's "natural" inclusion in that landmass) became apparent. In 1806, William George Browne still titled his travelogue Travels in Africa, Egypt, and Syria. Similarly, James Bruce in 1835 published Travels through part of Africa, Syria, Egypt, and Arabia. On the other hand, as early as 1670 John Ogilby under the title Africa published "an accurate Description of the Regions of Egypt, Barbary, Libya, and Billedulgerid, the Land of Negroes, Guinea, Æthiopia, and the Abyssines, with all the adjacent Islands, either in the Mediterranean, Atlantic, Southern, or Oriental Seas, belonging thereunto".

The usual line taken to divide Africa from Asia today is at the Isthmus of Suez, the narrowest gap between the Mediterranean Sea and the Gulf of Suez, the route today followed by the Suez Canal. This makes the Sinai Peninsula geographically Asian, and Egypt a transcontinental country. Less than 2% of the Egyptian population live on the Sinai Peninsula, and hence Egypt, even though technically transcontinental, is usually considered an African country entirely and not partly Asian. But when discussing the geopolitical region of the Middle East and North Africa, Egypt is usually grouped with the Western Asian countries as part of the Middle East, while Egypt's western neighbor Libya is grouped with the remaining North African countries as the Maghreb. Both are members of the African Union. However, the geological boundary is located further to the east, along the Gulf of Aqaba and the Dead Sea Transform, with the Sinai Peninsula, Israel, Palestine, much of Lebanon and the coastal governorates of Syria being located on the African Plate.

The Comoros, Madagascar, Mauritius, and Seychelles are island countries in the Indian Ocean which are nominally associated with Africa. The Socotra Archipelago may be considered African as it lies on its continental shelf; however, it is politically a part of Yemen, an Asian country.

The African and European mainlands are non-contiguous, and the delineation between these continents is thus merely a question of which islands are to be associated with which continent.