Introduction. Déjà vu (DV, from French “already seen”) is an aberration of psychic activity associated with transitory erroneous perception of novel circumstances, objects, or people as already known.Aim. Investigation of clinical and diagnostic significance of derealization episodes in epilepsy.Materials and methods. The study involved 166 individuals (mean age 25.2 ± 9.2 yrs; 63.2% women). DV episodes were characterized and compared in groups of healthy volunteers (n = 139) and epilepsy patients (n = 27). The subjects participated in a survey concerning déjà vu characteristics and in a long-time ambulatory EEG monitoring (12–16 h).Results. In epilepsy patients, DV episodes were equally frequent in cryptogenic and symptomatic focal epilepsy, occurred in combination with nearly all types of seizures, and could occur both as an aura and as an individual seizure. The major clinical features distinguishing DV in healthy subjects from DV in epilepsy patients were the frequency and emotional perception of DV episodes, and preceding fear. A critical diagnostic marker is the dynamics of DV characteristics: an increase in frequency and duration, negative emotional background. In EEG, DV episodes in patients began with polyspike activity in the right temporal lobe and, in some cases, ended with slow-wave theta-delta activity in the right hemisphere.Conclusion. Our combined clinical and electrophysiological investigation identified two separate DV types: epileptic Déjà Vu characteristic of epilepsy patients and equivalent to an epileptic seizure, and non-epileptic Déjà Vu occurring in healthy individuals, which is basically a psychological phenomenon.