Summary Medical as well as paramedical treatments should be evaluated by scientific methods. This systematic review focuses on the effects of voice therapy, excluding pharmacological or surgical treatments. In general, statistically significant positive but modest and varying therapy effects are found. Many of these effect studies cope with diverse methodological problems. Furthermore, the conclusions of most studies cannot be generalized easily or compared to one another. As a consequence, many issues in the field of effects of voice therapy have yet been unanswered.