This chapter reviews the general situation, pointing out the areas that have the most relevance to adolescent suicidal behavior. There appears to be an intimate relationship between the serotonergic parameters of aggression, anxiety, and suicidal behavior, especially in young people. Based on genetic studies and findings relating serotonin to suicidal behavior, some researchers have suggested that the gene coding for the rate-limiting enzyme in serotonin metabolism, tryptophan hydroxylase (TPH), is a candidate gene for suicidal behavior. Regardless of the etiology of suicidal behavior in young people, suicide attempts are extremely rare before puberty. The influence of hormonal factors on suicidal behavior has not been well studied. There has been a great deal of interest in the relationship between carbohydrate (CHO) metabolism, 5-HT systems and aggression. The development of operational criteria for nosological diagnosis has enabled biological researchers to focus on more clearly defined disorders in their search for biological markers.