This Note discusses the principles behind design decisions and procedures demanded by the rigours of a mountainous environment. The geographical areas addressed are geologically young fold mountain regions in humid tropical and sub-tropical climatic zones. However, most of the principles in the Note would be applicable in any steep, topographically complex, unstable terrain. The main subjects covered are terrain hazards, route planning and site investigations, drainage design, and slope stabilisation and protection. The Note emphasises the risks associated with inappropriate design and poor construction practice, and describes procedures and techniques that are safe and economical. The Note is aimed principally at design engineers, but it will also be of value to all those involved in site investigations and contracting.