Substantial progress has been made over the last several years in the development of protocols for the isolation of large numbers of dendritic cells (DCs) from different tissues and their short-term culture. Indeed, several stable DC lines and clones have been established from various tissues of mice and humans, providing useful experimental tools for studying the biology of DCs at both molecular and biochemical levels and for the establishment of new DC-based immunotherapies. In this chapter, we will describe the development of long-term DC lines that maintain the growth factor dependence and their immature functional state, thus providing a unique opportunity to study the mechanisms of the initiation of the immune response to infectious agents.