A complementary good (a complement) is a good whose consumption depends on consumption of another good. Both goods tend to be consumed together, i.e., each complement is necessary for the use/ consumption of the other (there is joint demand/ complementary demand for both). Examples of complementary goods include printers and ink cartridges, tea and sugar, cars and petrol, mobile phones and SIM cards, hardware and software, etc.

When demand for one good increases, demand for the other will also increase and vice versa. Typically, a complementary good would have no use if it is purchased alone (i.e., without the other complementary good). When the price of one good rises, demand for the other is likely to go down (along with the demand for the first), and vice versa.