This chapter discusses the quantitative information concerning attractant concentrations and gradients present in commonly used assays, obtained by the solution of the diffusion equation for a number of assays of interest. This chapter also considers the bridge visual assay, the Millipore assay, and the under agarose assay in both cylindrical and linear geometries. This chapter also analyzes the effect of cell degradation of attractant in the latter system, for it has experimentally found it to have a potentially significant influence on assay behavior. The response of cells to a chemotactic factor depends on both the concentration and the concentration gradient of the factor. Therefore, in assays designed to experimentally investigate the chemotactic behavior of cell populations, it is extremely important to know what attractant concentrations and concentration gradients are present in the assay system. In current assays, this means that the attractant concentration profiles must be described as a function of time and spatial position throughout the field of cell movement.