Metallic materials have a hierarchy of structures ranging in scale from nm to mm. Corresponding model constructs range from coarse-grained atomistics, microscopic phase field models, and dislocation field models, to discrete dislocation dynamics, statistical continuum dislocation models, and on up to mesoscale generalized continuum models of gradient, micropolar or micromorphic type, as well as local continuum crystal plasticity that can be applied over many grains. We discuss concurrent and hierarchical multiscale model transitions in space and time, distinguishing between coarse-graining and spatial domain decomposition approaches for lower scale models, as well as model order reduction for mesoscale levels and higher. In terms of bridging length and time scales and modeling formalisms, the practical importance of two-scale transitions between models of differing fidelity and/or resolution is emphasized, with consideration of uncertainty. We summarize some of the long-standing gaps in modeling dislocation plasticity in crystals and polycrystals.