An interpretation of use-wear marks on metal artifacts is developed from the principles of metal cutting and brittle fracture and applied to surficial markings and microstructural damage on bronze tools from Machu Picchu and environs. Most of the tools have blunt edges, relatively low tin contents, and were not work-hardened before use; they appear to have been designed for work that involved breaking chips from hard, brittle material. Use-wear marks on these tools are interpreted as due to sliding contacts and impacts with rock. One tool with a relatively sharp edge has a higher alloy content than those with blunt edges and has been work-hardened; it appears to have been designed for cutting wood and use-wear markings suggest it was so used. A long bronze bar carries markings that suggest use by stonemasons. Many of the tools are broken and study of their microstructures shows that the bronze used has poor mechanical properties because of porosity and bands of sulfide inclusions.