The chapter covers the competencies for defendants to participate in court proceedings, including competency to stand trial and competency to waive Miranda Rights. Also covered are the procedures for competency restoration for those deemed incompetent to stand trial. Thorough definitions provide readers with accurate explanations of one’s capacity and how it differs from the assumption of knowing “right from wrong.” The information in this chapter provides justice system practitioners with more knowledge on competency to support the needs of defendants with mental illness and disabilities. The chapter concludes with considerations and recommendations from experts in the field to help practitioners, particularly attorneys, work more effectively with clients and better collaborate with mental health professionals.