WITHIN SCANDINAVIAN JURISPRUDENCE, THE LEGALITY PRINCIPLE HAS BEEN DEFINED AS AN OBLIGATION IMPOSED ON THE PROSECUTOR TO TAKE LEGAL ACTION IN EVERY CASE WHERE THERE IS NO LEGAL GROUND FOR REFRAINING FROM PROSECUTION. THE OPPORTUNITY PRINCIPLE, ON THE OTHER HAND, AIMS AT GIVING THE PROSECUTOR SOME DISCRETION IN WHETHER OR NOT TO PROSECUTE. THE SAME PRINCIPLES APPLY FOR THE POLICE IN THEIR DETERMINATION OF WHETHER TO MAKE AN ARREST. FACTORS AFFECTING POLICE INVOLVEMENT IN ACTION AGAINST CRIMINALS CAN BE RESEARCHED ACCORDING TO THE FOLLOWING BROAD CATEGORIES OF CRIMES: CRIMES WITH A VICTIM, CRIMES UNDER THE JURISDICTION OF PUBLIC AGENCIES OTHER THAN THE POLICE OR PROSECUTOR, AND VICTIMLESS CRIMES. IN RESEARCHING POLICE INVOLVEMENT IN DEALING WITH CRIMES WITH A VICTIM, ATTENTION SHOULD BE GIVEN TO WHY A VICTIM DOES OR DOES NOT FILE A COMPLAINT WITH THE POLICE. THIS WOULD INVOLVE DETERMINING WHY CRIMES AGAINST CORPORATE BODIES MAY NOT BE REPORTED TO THE POLICE. A NUMBER OF CRIMES ARE FIRST REPORTED TO SOCIAL OR PUBLIC AGENCIES CONCERNED WITH CERTAIN LAW VIOLATIONS (INSURANCE FRAUD, TAX FRAUD, ECOLOGICAL VIOLATIONS). CASES REPORTED TO THESE TYPES OF ORGANIZATIONS OR AGENCIES ARE OFTEN NOT FORWARDED TO THE POLICE FOR ACTION, EVEN WHEN IT WOULD BE APPROPRIATE. RESEARCH SHOULD DETERMINE WHY THIS IS SO. REGARDING VICTIMLESS CRIMES, RESEARCH SHOULD DISCOVER WHY THE POLICE GIVE PRIORITY TO ACTION AGAINST CERTAIN TYPES OF VICTIMLESS CRIMES WHILE NEGLECTING OTHER TYPES. IN ADDITION TO RESEARCHING ISSUES AFFECTING POLICE INVOLVEMENT IN THE HANDLING OF CERTAIN CATEGORIES OF CRIMES, POLITICAL INFLUENCES AFFECTING POLICE SELECTION OF LAW ENFORCEMENT ACTIVITY SHOULD BE IDENTIFIED AND ANALYZED. A DIAGRAM ILLUSTRATED THE TEXT. (RCB)