This Nature Materials Review outlines the growing importance of picometer scale structural motifs for control and design of functional materials. The picoscale is the ultimate length scale for materials engineering via subtle distortions of atomic-scale structure: any smaller, and the resulting changes of properties are too small to matter; any larger, and bonds are entirely broken and one is discussing a different material. The work reviews picoscale materials science and engineering on a number of different classes of materials systems and explains why this decade marks a special time for picoscale science since multiple tools (theoretical and experimental) have converged to permit understanding and designing materials in a closed-loop cycle between theory and experiment. DOI 10.1038/natrevmats.2017.60