As a consequence of the widespread criticism leveled against the classification of somatoform disorders and especially the somatization disorder according to DSM-IV and ICD-10, a fundamentally revised diagnostic category was introduced by DSM-5 with the new main diagnosis Somatic Symptom Disorder. Substantial modifications concern the inclusion of somatic symptoms despite the existence of a medical cause and the inclusion of psychological symptoms that are required for making the diagnosis. In view of these changes, the dissertation aimed to empirically validate the diagnostic criteria of the Somatic Symptom Disorder and to study the clinical implications of the revision. Based on a sample of the German general population (N = 321), the modified somatic criterion A and the new psychological criterion B of the Somatic Symptom Disorder were evaluated using a longitudinal approach with a 1- and 4-year follow-up. Although the modification of the criterion A is confirmed by the present findings showing a comparable clinical relevance of medically explained and medically unexplained symptoms and due to the limited reliability of such a distinction, the inclusion of somatic symptoms with a medical cause is currently questionable given the state of research. Even though the inclusion of psychological diagnostic criteria is an essential change in general, the symptoms specified in criterion B were not the most valid in the context of somatoform disorders compared to other psychological symptoms with potential use for classification. Based on an ambulatory assessment approach (N = 28), the possibility of classifying functional syndromes as Somatic Symptom Disorder was investigated using the example of fibromyalgia syndrome. In principle, the classification of the fibromyalgia syndrome as a mental disorder might be considered because the psychological criterion B of the Somatic Symptom Disorder was frequently fulfilled. In form of a review article, the applicability of various diagnostic instruments developed for somatoform disorders was discussed for the assessment of the Somatic Symptom Disorder. None of the existing instruments using self-ratings or ratings by others turned out to be appropriate without specific adaption. In conclusion, the revisions of the Somatic Symptom Disorder in DSM-5 imply basic improvements compared to the often criticized somatization disorder according to DSM-IV and ICD-10. However, scepticism concerning the optimal definition of the classification criteria still remains. Further empirical investigations of the Somatic Symptom Disorder are important due to the fact that an enormous prevalence of the diagnosis in the health care system is expected. Not only science will benefit from a reliable and valid classification, but primarily also practice. There are many open questions regarding the continuity of the research field and the generalizability of the knowledge on somatoform disorders as well as the revision of the forthcoming ICD-11.