The recognition that lymphocytes existed in different varieties and that lymphoid organs were important for their differentiation greatly influenced immunological research. The growing awareness that started in the mid-fifties of the previous century has shifted the emphasis of immunology from a molecular, mostly serological science to the cell-oriented modern immunology of today. Matters such as hematopoietic differentiation, cell-cell interaction, cellular activation, as well as migratory behavior of hematopoietic cells received much attention and deepened our insight in the immune system. The relatively recent generation of mutant mice lacking lymphoid organs prompted the realization that the organogenesis of lymphoid organs could be dissected at the cellular and molecular level. Now we can distinguish several phases of development for lymphoid organs, and can assign molecules and cells to be essentially involved in these phases. Future research will identify additional molecules and cells required for the formation of the various lymphoid organs, because the picture is not complete yet.