Dissociative identity disorder (DID) belongs to the complicated issues of psychiatry and psychology areas. The specificity of the disorder and its clinical picture imply numerous difficulties in the diagnosis and treatment process. The diagnosis of DID can also have significant legal consequences, especially in the context of criminal liability or the general ability to be a witness. Thus, DID is an interdisciplinary problem. In practice, DID is rarely diagnosed, although it is estimated that it occurs in about 1% of the general population. In many cases, the period from the first contact with the healthcare system to a correct diagnosis exceeds several years (on average, 6.7 to 8 years). The average misdiagnosis rate is 2.8 per patient. The lack of a quick and proper diagnosis makes it impossible to undertake adequate treatment, which extends the entire therapeutic process, affects its effectiveness and significantly increases costs. There is no doubt that in educating psychiatrists and clinical psychologists, greater emphasis should be placed on correctly detecting dissociative symptoms and the use of adequate diagnostic tools. The aim of this article is to present and identify the main problems that DID implies in the diagnostic and therapeutic (psychological and psychiatric) areas. The article discusses the existing diagnostic tools, the issues of comorbidity and the causes of incorrect diagnoses. The issues of false-positive diagnoses and difficulties in differentiating patients with DID from simulators were also discussed. The primary mistakes made during the therapy, such as the strategy of minimization or the actions leading to multiple therapist disorder, were analyzed. Legal aspects will be presented in a separate article.