Long-term bone marrow cultures mirror many aspects of in vivo mammalian hematopoiesis. Thus, these cultures have been used widely to analyze the complex interactions that regulate hematopoietic differentiation. Hematopoiesis in vivo and in vitro is dependent on stromal cells, a mixture of support cells. In the past years numerous clonal stromal cell lines derived from murine and human tissues have been isolated and characterized. The stromal cell lines have proved to be invaluable tools for the elucidation of the molecular and cellular signals that govern differentiation and self-renewal of hematopoietic cells. This review describes the salient features of arrow cultures with a focus on the isolation and characterization of stromal cell lines. We summarize how stromal cell lines have been crucial tools for the detection, isolation, and maintenance of rare pluripotent stem cells and B lineage cells. Intriguing questions about the nature of stromal cells, their requirements for growth and differentiation, and their histogenic origin remain areas for further investigation.