Whether damage is localized or delocalized in a composite, the composite’s fracture toughness when localization occurs can be controlled to a much greater extent than hitherto exploited by properly choosing the composite’s internal geometry. Delocalization and high toughness are both favoured by building in systematic defects and lock-up mechanisms. Widespread defects make available arbitrarily many sites at which energy may be absorbed by non-linear behaviour. Lock-up mechanisms cause local hardening following local damage, which drives subsequent damage to initiate elsewhere, possibly leading to damage delocalization. In brittle-brittle composites, these mechanisms may be the best hope for achieving toughness values similar to those of alloys. Illustrations are taken from recent research into woven composites with three-dimensional reinforcement and new work on model composites.