Seismic tomography reveals 2 extensive regions in the lowermost mantle, beneath Africa and the Pacific, that exhibit lower-than-average seismic wave speeds. These regions have been named the Large Low Shear Velocity Provinces (LLSVPs), and they have spatial scales on the order of 1000s and 100s of kilometers in width and height, respectively. Discovering their cause remains an important challenge in the deep Earth community because recognizing what they are has important, first-order implications toward understanding the nature of global mantle convection and therefore, heat and chemical transport and evolution through time. At about an order-of-magnitude smaller scale are Ultra Low Velocity Zones (ULVZs) that reside on the core-mantle boundary. ULVZs are typically up to 100s of kilometers laterally and only 10s of kilometers vertically. We don't know what LLSVPs and ULVZs are, and the primary question is whether they are thermal or compositional features, and/or both. In any case, there is every reason to suppose that they are linked by a dynamical relationship, and by better understanding one, we can discover more about the other. Here, we review the observations associated with LLSVPs and the various conceptual mantle models that the community is debating regarding their cause. ULVZs, as they may relate to the larger LLSVPs, are also reviewed, and dynamical linkages between the two are discussed. Better understanding both LLSVPs and ULVZs promises to provide critical insight into global scale mantle convection and therefore provide a foundation for understanding numerous other processes in the Earth's interior.