Composite cryotanks, or Composite Overwrapped Pressure Vessels (COPVs), offer advantages over currently used aluminum-lithium cryotanks, particularly with respect to weight savings. Future NASA missions are expected to use COPVs in spaceflight propellant tanks to store fuels, oxidizers, and other liquids for launch and space exploration vehicles. However, reliability and reusability of the COPVs are of concern, especially in cryogenic temperature applications; this limits adoption of COPVs in future reusable vehicle designs. The major problem with composites is the inherent brittleness of the epoxy matrix, which is prone to microcrack formation, either from exposure to cryogenic conditions or from impact from different sources. If not prevented, the microcracks can grow into larger cracks, leading to catastrophic failure and loss of function of the composite. Accordingly, materials innovations are needed to mitigate, as well as self-heal, microcrack damage in composite cryotanks. In Phase I we propose to demonstrate microcrack prevention and mitigation in composite test panels through the use of a novel nanocomposite matrix containing engineered nanoscale materials which will also enable self-healing of microcracks. Phase II will build upon the Phase I program in order to optimize the material design and to characterize the long-term durability of the scaled-up composite test panels.