The introduction of new materials such as advanced composites, leads to not only new attributes for aerospace structures, but also introduces new technological elements to be considered. Historically, structural aircraft design has progressed from wood structures (homogenous, anisotropic materials), to metallic structures (homogenous, isotopic materials), to advanced composite structures (homogenous, anisotropic materials). While the progression has come full circle so to speak, our technical knowledge in developing, understanding, and modeling the behavior of these materials has increased exponentially. For example, the Wright Brothers used wood structurally without understanding the nuances of anisotropy. As design experience changed to metallic structures, our understanding and modeling at the phenomenological level of homogeneity and isotropy in successful design practice became documented. With the introduction of advanced composite materials, the inherent anisotropy of such materials has led to new classes of failure mechanisms unlike previous experience with metallic materials. In addition to inherent anisotropy, such materials can be brittle, and the initiation and growth of damage remains to a degree as work in progress. This has led to a design approach which can be considered semi-empirical and relies on our continued and developing experience for design/certification. In this paper, we will explore the damage tolerance issues based on design requirements, current state of the art design and analysis, some selected examples, and concluding remarks.