Aggression has been consistently shown to have a large genetic component based on findings from behavioral genetic studies. However, candidate gene and GWA studies on aggression have not identified a significant genetic marker. This does not mean that there are no genes relevant for aggression, but possibly indicates a complex interplay between various genetic components and environmental factors/triggers in aggression. Additionally, distinct forms of aggressive behavior (e.g., reactive, proactive, direct /physical, indirect/relational) may differ in their underlying neurobiological mechanisms. The extent to which environment affects epigenetic processes may depend on genotype, as well as other factors such as age, sex, and developmental history. The nonlinear occurrence of aggression over the lifetime (. Petersen, Bates, Dodge, Lansford, & Pettit, 2015) hints the involvement of developmental processes in the manifestation of aggression. Also, pathological aggression is highly comorbid with numerous psychiatric disorders and the concept of aggression is very heterogeneous, which further increases the complexity in the search for genetic and epigenetic markers. Notably, while most clinical studies focus on antisocial personality disorder, (epi)genetic research in psychiatric disorders that have aggression as a core symptom (such as IED) is scarce. Multidisciplinary collaborative research of genetic and epigenetic factors at various ages throughout the lifespan in well-characterized clinical and nonclinical study samples is needed to further shed light on the specific genetic and epigenetic contributions to (different types of) aggression including IED.