Adoption

Adoption is an option in both The Sims, The Sims 2, and the Sims 3. It allows a couple (opposite- or same-sex) or a single Sim to adopt a child.

The Sims
Sim 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. If you don't choose for a long time you get a child.

It is possible to get a kid that was taken away by the social worker in another family. When a child that was taken away from another family is adopted, their old parents and siblings will remain on their family tree and they will have no family relationship with their new parents. This makes adopting toddlers and children closer to fostering than actual adoption.

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. This isn't completely true, they may or may not look like their new parents.

Tips
If you are adopting a baby, and if the baby wasn't one who was taken away from the social worker, then 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
Adoption is confirmed in the Sims 3. A new feature added to the adoption process is being able to choose the sex of the potential adopted child. Adoption also has no financial prerequisites.