This contribution constitutes a commentary and critical discussion of the HFCR Manual on International Law Applicable to Air and Missile Warfare produced by the Program on Humanitarian Policy and Conflict Research at Harvard University (HPCR Manual). It is largely based upon a report written by one of the authors for the Expert Group that drafted the HPCR Manual. The introduction first determines the status of the HPCR Manual and tackles some general questions about the relation between treaty law and customary law in air and missile warfare, as well as whether the same rules apply, on the one hand, to hostilities taking place on land, sea and in the air, and on the other hand, to both international and non-international armed conflicts. Following this, it then looks at the precautionary measures to be taken by the attacker. Before discussing the individual provisions contained in Article 57 of the 1977 First Additional Protocol to the Geneva Conventions (Protocol I), certain general questions on precautionary measures are addressed. Next, it details the precautionary measures recommended by Article 58 of Protocol I. For both sections, the methodology consists of first looking at each precautionary measure as foreseen in Protocol I and determining its status under customary law. The article then assess how each rule has generally been interpreted, and more importantly how each rule has been applied to air and missile warfare, discussing what specific problems exist, particularly in the context of air-to-air hostilities.