Lentiviral vectors offer unique versatility and robustness as vehicles for gene delivery. They can transduce a wide range of cell types and integrate into the host genome in both dividing and post-mitotic cells, resulting in long-term expression of the transgene both in vitro and in vivo. This protocol describes how lentiviral vectors can be produced, purified and titrated. High titer suspensions can be routinely prepared with relative ease: a low-titer (10(6) viral particles/ml) unpurified preparation can be obtained 3 d after transfecting cells with lentiviral vector and packaging plasmids; a high-titer (10(9) viral particles/ml) purified preparation requires 2 more days.