Structural polymers are susceptible to damage in the form of cracks, which form deep within the structure where detection is difficult and repair is almost impossible. Cracking leads to mechanical degradation(1-3) of fibre-reinforced polymer composites; in microelectronic polymeric components it can also lead to electrical failure(4). Microcracking induced by thermal and mechanical fatigue is also a long-standing problem in polymer adhesives(5). Regardless of the application, once cracks have formed within polymeric materials, the integrity of the structure is significantly compromised. Experiments exploring the concept of self-repair have been previously reported(6-8), but the only successful crack-healing methods that have been reported so far require some form of manual intervention(10-18). Here we report a structural polymeric material with the ability to autonomically heal cracks. The material incorporates a microencapsulated healing agent that is released upon crack intrusion. Polymerization of the healing agent is then triggered by contact with an embedded catalyst, bonding the crack faces. Our fracture experiments yield as much as 75% recovery in toughness, and we expect that our approach will be applicable to other brittle materials systems (including ceramics and glasses).