The accurate quantification of functional vector particles within a given vector preparation or stock is of fundamental importance to virtually all potential gene transfer applications for any viral vector of interest. This accuracy is especially critical if anticipated clinical applications will require scale-up of vector production or if multiple different vector stocks will be required for serial transductions of target cell populations over the course of a series of experiments. Standard notations designating the quantity of functional particles in a vector preparation include infectious units (i.u.) per milliliter and transducing units (TU) per milliliter (1). These units are interchangeable and represent the number of functional vector particles per volume of vector containing supernatant. The quantification of infectious or transducing units per unit volume allows transductions to be conducted based on the number of vector particles added per target cell and will therefore be standardized yielding more reproducible results. The numerical relationship between vector particles utilized per single target cell is designated as the multiplicity of infection (MOI). Establishing an appropriate MOI for a given transduction will, in part, depend on the desired transduction efficiency or, to an extent, the approximate number of chromosomal integrants desired per cell. Accurate vector stock titers will allow for the establishment of controlled transduction conditions and will, therefore, reduce experimental variability and simplify the interpretation of results obtained.