Pokémon breeding/Games/How to breed

From Generation II onward, Pokémon can be bred by leaving two compatible Pokémon at the Pokémon Day Care (that takes two Pokémon) or the Pokémon Nursery.

Pokémon can be bred at Pokémon Day Cares or Pokémon Nurseries at the following locations:

Two Pokémon are compatible if they are of the same species (or share at least one Egg Group) and are of opposite genders; alternatively, breeding is usually still possible as long as one of them is, even if the other parent is genderless, and in fact, genderless Pokémon can breed only with Ditto. Pokémon in the egg group cannot breed in any way whatsoever, and Ditto cannot breed with itself.

In Generation II only, if the IVs of two Pokémon are the same, and the  IVs are either the same or differ by 8, they cannot produce Eggs. Because of how IVs are passed down from parents, this condition suggests the Pokémon are related.