All cancers emerge after a period of clonal selection and subsequent clonal expansion. Although the evolutionary principles imparted by genetic intratumour heterogeneity are becoming increasingly clear, little is known about the non-genetic mechanisms that contribute to intratumour heterogeneity and malignant clonal fitness. To address these important issues, we recently developed SPLINTR (single-cell profiling and lineage tracing), an expressed barcoding strategy that enables a detailed temporal and spatial assessment of clonal contribution to disease initiation and adaptation to various therapeutic challenges. In this session, I will discuss some of our recent work in a variety of haematological and solid malignancies that provide new insights into the roles of genetic and non-genetic processes that underpin malignant clonal fitness and enable therapeutic resistance.