Orthodontic tooth movement is induced my mechanical stimuli and facilitated by the remodeling of the periodontal ligament and alveolar bone. The remodeling activities and the ultimately tooth displacement are the consequence of an inflammatory process. Vascular and cellular changes were the first events to be recognized and described. With the advancement of research techniques an important number of inflammatory mediators, growth factors, neuropeptides and metabolites of arachidonic acid have been detected in the surrounding periodontal tissue. Most of them are produced in sufficient amounts to diffuse into the gingival crevicular fluid and their analysis has been used to evaluate the local cellular metabolism that accompanies bone remodeling process during orthodontic tooth movement. Gingival inflammation, discomfort or pain and root resorption are common undesirable side effects of orthodontic treatment. Gingival health is compromised during orthodontic treatment because of alterations in the composition of bacterial plaque and consequently the development of gingivitis, especially when fixed orthodontic appliances are used. Discomfort or pain is reported by most patients during the first day or couple of days of treatment and comes from the compression of periodontal ligament after application of the orthodontic force. As for root resorption, it has been reported that almost all roots, shortly after the application of orthodontic force present histologically small areas of surface resorption. In most cases this resorption does not decrease the functional capacity of the involved teeth, but in other cases the degree of resoprtion may become important, resulting in an unfavourable crown/root ratio and thus less periodontal support for the tooth, decreasing its longevity. In the present thesis, the following questions are addressed: • What are the clinical and biochemical characteristics accompanying the inflammatory process of periodontal tissues around teeth subjected to forces by fixed orthodontic appliances? • Does the composition of gingival crevicular fluid change in relation to the type of force exerted on the periodontium (tension or compression) during the early phase of orthodontic treatment? • Does a common orthodontic procedure such as the placement of elastic separators result in discomfort or pain to the patients and is there an association between the expression of pain-related molecules in the GCF and the perceived intensity of pain? • Do the standardized clinical periodontal parameters obtained during orthodontic treatment could be related to the cervical root resorption observed after tratement? • What is the diagnostic accuracy of digitized periapical radiographs in detecting orthodontically induced apical root resorption? • Does the use of orthopantomograph can help the clinician to make the decision on continuation and /or possible modification of orthodontic treatment because of orthodontically induced root resorption?