It is time to take inventory of the knowns and unknowns about policy tools. The study of this field dates back to Lowi and others who have developed many typologies and theories in classic works such as Hood, Salamon, Linder and Peters, Peters and van Nispen, Schneider and Ingram, Lascomes and Le Gales. Furthermore, policy instruments have become a relevant topic in many policy fields due to their theoretical and empirical relevance (overall, policies are made and pursue their goals through policy instruments). This is important work that is linked closely to current research on policy design but, despite much effort, many fundamental issues remain unknown or under-studied with respect to the topic. The current paper examines four clusters of basic issues in the field which require further research.