Identifying the genes and pathways that regulate self-renewal and differentiation in somatic stem cells is a central goal in stem cell and cancer biology. Here, we describe a method for RNAi-based screens in primary human hematopoietic stem and progenitor cells. These cells are suitable targets for complex, selection-based screens using pooled lentiviral shRNA libraries. The screening approach is a promising new tool to dissect regulatory mechanisms in hematopoietic and somatic stem cells, in general, and may be particularly useful to identify gene targets and modifiers that can be further exploited in strategies for ex vivo stem cell expansion.