This paper presents a brief review of markers and their potential uses in the study and treatment of affective illness. Some of these include the use of markers as aids in: 1) diagnosis, 2) selection of medication, 3) prediction of relapse, and 4) prediction of suicide. The use of combinations of markers to improve accuracy and the use of markers during the well-state will be discussed. A review of the literature suggests some promising candidates as markers of affective illness including the serotonin metabolite, 5 HIAA; 3H-imipramine platelet binding; REM induction using M-1 muscarinic agonists; and melatonin. Finally, a strategy using gene mapping techniques as a possible method of detecting additional markers is presented.