Etiology of transversal malocclusions may be genetic, muscular or environmental. Early treatment of these malocclusions has a good short- term and long-term prognosis. The objective of this review was to create clinical guidelines, following a systematic and logical order, which could help practitioners in the search for a proper diagnosis and etiological factors. The first step of the guidelines is anamnesis and then a clinical examination that includes facial and occlusal analysis must be performed. Analysis of cast models and radiographs (panoramic, posteroanterior and lateral cephalogram) complements the assessment. Finally, a description of clinical regulations is presented which may be of help when determine the need of an expansion treatment, as well as the prediction for space increase in the lower arch through out the treatment.