Criminal law and criminal procedure law provide the normative context for crime investigation by public police organizations around the world. The European continental model provides that state action is not permissible unless regulated by statute and as a result, legislators enact laws to proscribe and limit investigative activity. Between legal systems, there are some areas of similarity, such as pre-trial investigations, state led prosecutions, and disclosure regimes. While there is no full harmonisation between criminal justice systems across the globe, in some regions like the European Union a gradual merging process can be discerned, primarily due to the acknowledgement of transnational crime and terrorism. Except for scrutinizing the development of criminal law, the author analyses a number of cases and highlights the salience of professional requirements for the often complex task of criminal investigation against the normative tapestry of the national and international Rule of Law.