We present a review of phenotype-genotype correlation and the genetics of photosensitivity. The photoparoxysmal response in EEG (PPR) is still one of the best paradigms for exogenously triggered brain responses based on a genetic predisposition. The definition of the PPR phenotype requires multiple, precise methodologic guidelines. Individual factors such as age and gender but also other, unknown factors influence the expression of the PPR. For example, PPRs occur during adolescence and can disappear at a later age. As a consequence, it is difficult to assign nonaffected disease status correctly. Autosomal dominant inheritance has been found in clinical studies of relatives of PPR-positive epilepsy and nonepilepsy subjects. Genetic heterogeneity of the PPR is obvious because the PPR also can be evoked in a number of autosomal recessive diseases. PPR is most commonly associated with idiopathic generalized epilepsies (IGEs) such as juvenile myoclonic epilepsy (JME). This comorbidity suggests that a genetic factor involved in photosensitivity also may influence the susceptibility for JME. Finding the gene for PPR also might represent a step forward in unraveling the genetic background of JME. The search for the genetic factors causing PPRs should focus on the genes affected in such epilepsies, such as genes (coding) for ion channels and neurotransmitters and their receptors. The expression of defined proteins with as-yet-undetermined functions, is changed in a few types of epilepsies with a mendelian mode of inheritance. These additional genes and the human equivalents of the genes found to be mutated in animal models also are candidates for molecular genetic studies of the PPR.