Dendritic cells (DCs) are versatile controllers of the immune system, best known for their potent ability to initiate adaptive immunity. Traditionally, DCs have been defined on the basis of cell morphology, expression of specific markers and select functional attributes such as the ability to migrate to T cell areas of secondary lymphoid organs and activate T lymphocytes. However, such properties are not qualitative and often change in conditions of inflammation or infection. Phenotypic-based and function-based definitions can therefore lead to difficulties in cell identification. Here we review other approaches to try and solve questions of DC lineage attribution with an emphasis on recent insights arising from our increased understanding of DC ontogeny and differentiation.