Adoption

Adoption is an option in both The Sims and The Sims 2. It allows a couple (opposite- or same-sex) or a single Sim to adopt a child.

The Sims
Sims couples get calls from adoption centers randomly asking if they want to adopt a baby. One person cannot adopt a child.

The Sims 2
Sims can adopt a child over the phone if they call Service and then choose "Adoption Center". They could only do this however if the Social Worker had not taken one of their kids. The household must have at least 3000 simoleons on hand (though adoption itself does not cost any money) and below 8 household members to adopt.

If you are approved the Social Worker will bring your new child the next day at 10 AM. To make sure someone is there to receive the child, it's best to call the day before a Sim's day off.

At 10 AM the Social Worker will arrive and a screen will pop-up asking you if you wish to adopt a baby, toddler, or child. This is the only option you have when adopting.

It is possible to get a kid that was taken away by the social worker in another family.

If a child or children are taken away by a social worker in one family, they are first in line to be adopted by another family.

Some people say that adopted children will look like their new parents.

Hints:

If you are adopting a baby, and if the baby wasn't one who was taken away from the social worker, than the social worker will come into your house and give you the baby, and you get to name him/her. Be warned though: The baby has maxis-created genetics, so if you want a really cute/non ugly Sim, then adopt a toddler or a child, so you will know what they look like.

The Sims 3
We know that adoption will be in the Sims 3, as well, because Adriana Lothario and Mary-Sue Pleasant must be adopted.