Selective mutism (SM) describes inhibited, anxious and withdrawn children who are persistently mute in important social situations despite ability to talk. SM may cause great suffering and create problems, both socially and educationally, and problems per sist. SM is associated with social anxiety, neurodevelopmental delay / disorder and bilingualism. Childhood prevalence is about 0.7%-0.8%. Adequate assessment and treatment of SM is seldom provided in the child and adolescent mental health services (CAMHS ). SM is considered to be hard to treat, and randomised treatment studies are lacking. This RCT study will examine the effect of a manualised assessment and treatment for SM after three and six months. The treatment manual, developed by the research group , consists of home- and kinder garden / school based interventions including behaviour techniques and psychoeducation. Defocused communication is a general treatment principle. Comorbidity, including social anxiety and neurodevelopmental delay / disorder, predictors of treatment outcome and maintenance of effect will be examined. A pilot study was conducted to ensure the feasibility of the planned effectiveness study. Seven children, aged 3-5 years were included. Six has started treatment, and all talked in the kinder garden within the first three months. The present study will have a RCT design with 1. Manual based interventions for six months compared to 2. Waiting list controls (three months) who then receive the same manual based interventions. Childr en aged 3-9 years consecutively referred to the school psychology services or the CAMHS in Oslo and Eastern Norway, will be included. Expected N = 24, based on the pilot study, is a sufficient sample size to answer our primary research question. The treat ment will be given by a therapist from the research group or by a local clinician, under supervision. The study can add essential knowledge on treatment of SM and make effective treatment available to clinicians.