There has been significant debate over the identity of cancer stem cell populations in acute lymphoblastic leukaemia (ALL), with different groups reporting seemingly contradictory results. The latest findings suggest that tumour-propagating capacity is found within a high percentage of ALL blasts and that these cells have diverse immunophenotypes, which suggests that ALL follows a stochastic cancer stem cell model - as opposed to a hierarchical model. Recent data add a layer of complexity to the tumour evolution process by showing that the leukaemia-propagating compartment consists of multiple genetically diverse subclones related by Darwinian-style evolutionary trees. Differences in the cell of origin may also affect tumour development. In this article, we discuss the applicability of cancer stem cell models to ALL in the context of these recent findings.