Plastic deformation exhibits a strong dependence upon size at and below micrometer length scales. To model micrometer scale plasticity we propose a theory called mechanism-based strain gradient plasticity (MSG), which connects microscale dislocation interactions to mesoscale plasticity via a hierarchical framework linking Taylor's dislocation hardening model to strain gradient plasticity. We distinguish the microscale, at which dislocation interaction is considered, from the mesoscale, at which the plasticity theory is formulated the MSG theory differs from all existing phenomenological theories of plasticity in its mechanism-based guiding principles. We show that the MSG theory gives predictions in agreement with microindentation, microtorsion, and microbending experiments.