Over the past few decades both the demand for knowledge as well an increase in the application of composite materials has boomed. This has led to an intensive focus by the research community to gather information and validate the use of such materials. Sandwich composites have been a particular interest and an intense demand for further understanding was sparked by the catastrophic rudder failure of Air Transat 961. This thesis focuses on understanding fracture mechanics and damage tolerance within sandwich composites. Facesheet disbond has severe impacts on the material systems strength and stiffness, and large disbonds can often lead to catastrophic component failure. Understanding how these disbonds grow is paramount to recognizing the limitations of sandwich composites. This thesis has several objectives. First, determine the fracture toughness of various sandwich composite material systems under going quasi-static loading. Second, determine how variations within the material systems such as facesheet thickness, core type, cell size and core density effect the results. Third, determine how the failure modes such as adhesive, pullout and core alter the results. Fourth, determine how fluid ingression effects fracture toughness. Fifth, provide baseline data for further testing and modeling.