Adherence to treatment in mental health is generally regarded as central for optimizing recovery. However, poor adherence of patients with psychiatric disorders can have devastating consequences such as relapse, rehospitalization and poor quality of life. Adherence problems are likely to be multi-determined and related to different factors in different individuals. Combining subjective and objective quantitative assessment with a more qualitative interview might help assess adherence better in patients with mental illness to determine the presence of adherence problems and investigate their cause. The therapeutic relationship between patient and clinician has been found to be important for treatment adherence. Adherence problems should be regarded not as the result but as the process of the treatments to facilitate the therapeutic alliance. Clinicians and patients need to cooperate with each other in a model of shared decision-making to choose the best treatment option for the specific individual. Psychoeducation might provide information about the risk and benefit of the medication to patients and their family. The collaborative approach is helpful for the clinicians to have an open dialogue about what patients think and perceive about the treatment.