Individuals with mental illnesses are overrepresented at every stage of the criminal justice process. In response, many jurisdictions have developed a range of policy and programmatic responses that depend on collaboration among the criminal justice, mental health, and substance abuse treatment systems. A critical component of this collaboration is information sharing, particularly about the health and treatment of people with mental illnesses. However, legal and technical barriers often prevent a smooth exchange of information among these systems. Understanding the legal framework of information sharing is the crucial first step for jurisdictions seeking to design and implement effective criminal justice-mental health collaborations. This guide, supported by the U.S. Department of Justice, Bureau of Justice of Assistance, supports that first step by introducing how Federal and State laws are likely to influence practitioners' responses. Federal law shapes what is permissible at the State or local level, primarily through the basic privacy rules for "protected health information" under the Health Insurance Portability and Accountability Act of 1996 (HIPPA) and other privacy laws. The guide is organized into two parts. The first part focuses on each type of practitioner likely to be involved in criminal justice-mental health collaboration: behavioral health care, law enforcement, courts, jail and prison, and probation and parole. The second part, "Working with Privacy Laws" provides practical advice on the enforcement of HIPAA and other privacy laws. Notes