The study represents the first attempt to formulate the basic concepts and terms in the course of exploration of negative effects of psychological treatment in the Czech psychotherapy. The study focuses on: unwanted effects, adverse events, side effects and unsuccessful psychotherapy. The variables on the client’s side and the factors on the therapist’s side are in the play concerning the negative events and consequences in therapy. In psychoanalytic tradition the negative therapeutic reaction had been formulated in the past. Psychotherapist himself/herself might became a risky factor in psychological treatment. Saxon et al. (2017) explored obvious differences in drop-out among therapists. Today, we have first classifications of unwanted effects given by researchers in the field. The study presents classification recently offered by Schermuly-Haupt et al. (2018). In addition, the study gives a list of often emerged adverse effects of psychotherapy.