Michael Ashby’s early work illuminated problems in materials science — including dispersion hardening, the migration of particles through solids, dislocation nucleation, diffusional creep, grain boundary sliding, the deformation of cellular structures using physical insight to guide experiments and mathematical modelling. He has pioneered the important and fruitful concepts of deformation and fracture maps, which display in a single field the area of dominance of specific flow and fracture mechanisms. These are most useful to engineers, metallurgists, ceramists and geologists. More recently, his focus has been on systematic methods for materials selection to meet criteria of engineering design, environmental stewardship and sustainable development.