Bone represents a porous tissue containing a fluid phase, a solid matrix, and cells. The movement of the fluid phase within the pores or spaces of the solid matrix is referred to as interstitial fluid flow. Although much has been postulated on the effects of this fluid movement on cell behavior (see Section 22.5), relatively little is known about the interstitial fluid per se, the properties of the spaces and channel walls through which the fluid flows, or the nature of the flow under physiological conditions. Despite the paucity of concrete information on interstitial fluid flow in bone, much has been postulated on its role in physiological processes such as metabolism, as well as within the context of bone biology and mechanisms of growth, repair, and adaptation. This chapter reviews the “knowns” as well as the “unknowns” of interstitial fluid flow and should serve as an impetus for further research, so that the “knowns” will outweigh the “unknowns” in the future.