The idea that some types of behaviour are defined as criminal is such a familiar feature of modern societies that we rarely consider how this comes about. The way in which laws get made is important for investigators because the provisions of statutes, and the ways in which they are interpreted by judges, determine the type of information that must be located to bring offenders to justice. The law also lays down the rules by which evidence can be gathered by investigators and the standards required before a prosecution can be mounted. The nature of the task of criminal investigation is therefore largely shaped by the criminal law, and understanding the type of task that investigation is for practitioners requires an understanding of how laws are made. But every law cannot be enforced all the time and choices have to be made about what crimes police forces should focus on and the resources that should be applied to them. Furthermore, how investigators apply the law is considered to be important. Society expects them to operate to high ethical standards, to be civil, to be fair and to exercise discretion in the use of the powers that the law gives them. Setting the objectives for criminal investigation, allocating resources and defining the methods by which investigations are to be carried and how discretion is to be exercised is achieved through the development of policy. This chapter examines how law and policy come to be made, the complex relationship between them and how they influence investigative practice.