This chapter provides an overview of developmental issues related to side effects, and then reviews the most common adverse events and concerns related to mood stabilizers and atypical antipsychotics. There are clear developmental differences in adverse event expression or side effects risks when mood stabilizers and antipsychotics are used in children and adolescents. All the mood stabilizers and antipsychotic medications produce adverse events. Each mood stabilizer and each atypical antipsychotic medication has its own adverse events profile. Prescribing physicians need to be familiar with the spectrum of risks for each of these medications.