Response to psychopharmacologic drugs is genetically complex, results from an interplay of multiple genomic variations with environmental influences, and depends on the structure or functional expression of gene products, which are direct drug targets or indirectly modify the development and synaptic plasticity of neural networks critically involved in their effects. During brain development, the serotonin (5HT) system, which is commonly targeted by antidepressant, anxiolytic, and antipsychotic drugs, controls neuronal specification, differentiation, and phenotype maintenance. While formation and integration of these neural networks is dependent on the action of multiple proteins, converging lines of evidence indicate that genetically controlled variability in the expression of the 5HT transporter (5HTT) is critical to the development and plasticity of distinct neurocircuits. The most promising finding to date indicate an association between the response time as well as overall response to serotonin reuptake inhibitors (SSRIs) and a common polymorphism (5HTTLPR) within the transcriptional control region of the 5HTT gene (SLC6A4) in patients with depressive disorders. The formation and maturation of serotonergic and associated systems, in turn, are influencing the efficacy of serotonergic compounds in a variety of psychiatric conditions. Based on the notion that complex gene x gene and gene x environment interactions in the regulation of brain plasticity are presumed to contribute to individual differences in psychopharmacologic drug response, the concept of developmental psychopharmacogenetics is emerging.