Surrogacy is an arrangement whereby a woman agrees to become pregnant for the purpose of gestating and giving birth to a child for others to raise. She may be the child's genetic mother (the more traditional form of surrogacy), or she may be implanted with someone else's fertilized egg (gestational surrogacy).

Surrogacy is a method of assisted reproduction. In some cases it is the only available option for a couple who wish to have a child that is biologically related to them.

In traditional surrogacy the surrogate is pregnant with her own biological child, but this child was conceived with the intention of relinquishing the child to be raised by others; often by the biological father and possibly his partner, either male or female.

In gestational surrogacy the surrogate is pregnant via embryo transfer with a child of which she is not the biological mother. She may have made an arrangement to relinquish it to the biological mother or father to raise, or to a parent who is themselves unrelated to the child (e.g., because the child was conceived using egg donation and/or sperm donation).

Altruistic surrogacyis a situation where the surrogate is not receiving financial reward for her pregnancy or the relinquishment of the child (sometimes with the exception of medical expenses associated with the pregnancy or birth). Compare with Commercial surrogacy which is a type of surrogacy in which the surrogate is being paid for her pregnancy and the relinquishment of the child. It is typically combined with gestational surrogacy (see Commercial surrogacy).

A surrogate mother or birth mother is the woman who is pregnant with the child. The word surrogate, from Latin subrŏgare (to substitute), means appointed to act in the place of. The commissioning parents are the individual or persons who intend to rear the child after its birth.

There is a tendency now to limit the term 'surrogacy' to only mean 'gestational surrogacy'.