The presence of surfaces near a swimmer can impact dramatically its ability to generate propulsive forces for locomotion. In this eleventh chapter, we review theways in which boundaries influence cellular propulsion from a hydrodynamic standpoint. At the cellular level, both the distribution of cells and their swimming kinematics are affected. On smaller length scales, boundaries govern the ability of appendages such as cilia to produce net forces and flow. We first consider length scales much larger than those of the cells. We show how the method of images for hydrodynamic singularities can be used to demonstrate that long-range hydrodynamic interactions lead to the attraction of swimming cells by boundaries and to a change in the swimming kinematics of bacteria from straight to circular. We then examine the dynamics of swimming cells in shear flows and explain how the presence of a surface leads to cell reorientation and upstream swimming. We next revisit the waving sheet model near a boundary to show how increased friction impacts locomotion kinematics. We finish by zooming in to the sub-cellular level and addressing the role played by surfaces on force and flow generation by cilia.