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The influence of microcapsule diameter and crack size on the performance of self-healing materials is investigated. These epoxy-based ma-terials contain embedded Grubbs ’ catalyst particles and microencapsulated dicyclopentadiene (DCPD). Autonomic repair is triggered by rupture of the microcapsules in response to damage, followed by release of DCPD into the crack plane where it mixes with the catalyst and polymerizes. The amount of liquid that microcapsules deliver to a crack face is shown to scale linearly with microcapsule diameter for a given weight fraction of capsules. In addition, self-healing performance reaches maximum levels only when sufficient healing agent is available to entirely fill the crack. Based on these relationships, the size and weight fraction of microcapsules can be rationally chosen to give optimal healing of a prede-termined crack size. By using this strategy, self-healing is demonstrated with smaller microcapsules and with lower weight fractions of micro-capsules than have been possible in previous self-healing systems