Certain populations of patients require special considerations when lithium is prescribed. Children and adolescents have higher volumes of body water and more active renal glomerular filtration rates than adults. Their central nervous system is developing and therefore is vulnerable to the impact of substances, including medications such as lithium, that can cause side effects or adverse events. Elderly patients have less body water, slower metabolism, and often comorbid illnesses, so they also require close evaluation and monitoring when prescribed lithium. This paper examines the indications for, pharmacokinetics of, clinical uses of, and side effects of lithium in children, adolescents, and the elderly. Use of alternate mood stabilizers is also addressed briefly.