The concept of Insensitive Munitions was developed more than 40 years ago. The challenge for the munition designer was to produce munitions that gave the required performance but did not respond violently when subject to accidents or enemy action. Given that munitions contain metastable energetic materials which are by design able to release large quantities of energy in very short time-frames, coupled with the other constraints of reliability, service life, environmental impact, ease of disposal and cost it is easy to understand that this was and to some extent still is a major challenge. This paper provides a brief summary of the key technologies developed over the last 40 years and provides a set of design principles to facilitate the IM design process.