During development lymphoid tissue inducer (LTi) cells are the first hematopoietic cells to enter the secondary lymphoid anlagen and induce lymphoid tissue neogenesis. LTi cells induce lymphoid tissue neogensis by expressing a wide range of proteins that are associated with lymphoid organogenesis. Among these proteins, membrane-bound lymphotoxin (LT) $alpha1eta2$ has been identified as a critical component to this process. LT$alpha1eta2$ interacts with the LT$eta$-receptor on stromal cells and this interaction induces up-regulation of adhesion molecules and production of chemokines that are necessary for the attraction, retention and organization of other cell types. Constitutive expression of LT$alpha1eta2$ in adult LTi cells can result in the formation of a lymphoid-like structure called tertiary lymphoid tissue. In this review, we summarize the function of fetal and adult LTi cells and their involvement in secondary and tertiary lymphoid tissue development in murine models.