The titer of a lentivirus vector is often expressed in transducing units per milliliter. This is a functional titer that reflects the lentivirus’ ability to transduce a particular cell line under specific conditions. Transduction of other cell lines is likely to be different and will require optimization. 293T cells are used for production of lentivirus stocks, and they can be easily transduced with vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped lentivirus vectors. Consequently, this cell line is commonly used to determine the functional titer of lentivirus vector stocks produced with this envelope. For lentivirus vectors encoding fluorescent proteins under the control of promoters functional in these cells, titration can be performed using the limiting dilution method or a flow cytometry-based method. For lentivirus vectors lacking a fluorescent marker, or for those carrying promoters that may not be functional in 293T cells, titer can be determined either by real-time polymerase chain reaction (PCR) quantification of viral genomes in genomic DNA from transduced cells or a p24 ELISA-based assay.