Moiré patterns are widely observed in layered systems of weakly interacting 2D materials such as graphene, h-BN, MoS2, WSe2, etc. We explain these patterns using the concept of interlayer or van der Waals (VdW) dislocation arrays. We note that patterns and the defects appearing therein are electronic structure artifacts of the weak interactions between layers, locked into place by the strong in-plane interactions in the constituent layers; they are therefore topological states. We explain several experimentally observed Moiré phenomena, including the distinct Moiré patterns formed by different combinations of 2D materials, such as h-BN and graphene, on the same metal support layer. We also examine point and line defects in Moiré patterns, and explore their connection with defects in the constituent 2D layers. Finally, we examine the link between Moiré patterns in bilayer MoS2 and out-of-plane deformation. These examples inspire the idea of Moiré engineering, through which it is possible to design topological states for devices, with applications including electron transport, photovoltaics, and charge-based storage.