Most humans have two kidneys, one on the left and one on the right side of the spine in the back.

However, there are exceptions. Some people are born with only one kidney or lose one to surgery or injury, and a single healthy kidney is usually enough for a normal life. In kidney transplant patients, surgeons often leave the original kidneys in place, so a person can temporarily have three or even four kidneys (two non-working ones plus one or two transplanted).