Lithium carbonate has established itself as an effective therapeutic agent in primary affective disorders. As not all the patients with primary affective disorders respond to lithium therapy, it is necessary to identify responders prior to treatment. The important indicators of favourable lithium response include a definitive diagnosis of primary affective disorder, occurrence of less than four episodes of mania and depression within one year, psychotic features during both manic as well as depressive episodes, "grandiose-elated" picture during manic episodes; a family history of bipolar illness and response of affected family members to lithium treatment. While those with more than four episodes are not likely to respond to lithium therapy, those with episodes less frequent than once a year or two may not need prophylactic lithium. Among the depressed, hypersomnic depressed patients respond to lithium combined with a monoamine oxidase inhibitor. In addition to clinical predictors of response to lithium treatment, there are a number of pharmacokinetic, neurophysiological and biochemical indices which have been employed as supplementary predictors of response to lithium therapy.