The aim of this paper was to provide a literature review about the problems that can occur during orthodontic treatment. Using the PubMed database we collected items that would provide information regarding the direct consequences of the placement of an orthodontic appliance: coming to the discussion of the following topics: candida infections, the effects on the soft tissues, the effects on periodontal tissues and effects on hard tissues. The presence of appliances in the oral cavity increases the prevalence of people with candida, specifically the species Candida Albicans is the most frequently isolated. The balance between the clearance of the microorganism, the colonization and the state of candidiasis depends both on the virulence of the fungus, and the competence of the host immune system. On soft tissues, cases of ulceration of the upper jaw by a rapid palatal expander and pyogenic granuloma due to quad helix appliance have been reported. The second one is mostly observed on vestibular gingiva. The first one was found, however, in patients suffering from diabetes mellitus type 1 because of the tissue modifications induced by this pathological condition. The more severe periodontic effects are those caused by incorrect use of orthodontic elastic separators. Finally, the White Spot Lesions are the direct consequences of a wrong conditioning of enamel when attaching the bracket. They represent a first stage of caries in the positioning area of the bracket. The orthodontist is required to intercept these issues not to affect the success of the treatment.