M ore than a decade after the beginning of the genomic era that has completely reshaped medical and life sciences, we are now entering a new equally revolutionary phase, the single-cell era. To date, most of our biological knowledge has been obtained by populationlevel studies, based on the fundamental assumption that every cell within a defined population contributes equally to its characteristics and behaviour. As our understanding of the dynamics of cell populations improves, it is becoming increasingly evident that this assumption is not true in many cases. An important example is presented by the immune system, the function of which can be profoundly influenced by the activity of relatively rare cells such as antigen-specific T and B cells. The development of a plethora of single-cell technologies during the last few years has provided us with powerful tools to finally tackle this issue in a systematic way. These technologies span from transcriptomics and proteomics to imaging approaches enabling the study of living cells over time.In this Special Feature, we present a series of reviews that highlight how advances in both instrumentation and computational methodologies are transforming the study of cell biology and immunology. The inherent strength of these approaches is in the ability to resolve cellular heterogeneity at great detail in an unbiased way. This obviously opens up many new possibilities in the analyses of rare cell populations, as we discuss in our contribution. 1 Importantly, single-cell resolution has also proved highly informative in studies on developmental and differentiation processes, during which cells can typically be found in a continuous gradient of transitional states. In the review by Cvejic, 2 these principles are illustrated in the study of haematopoiesis. A conceptually related process with tremendous immunological significance is the diversification of T and B lymphocytes into numerous functionally specialized effector and memory cell subpopulations. The first wave of research reports has already indicated the great promise of single-cell approaches in dissecting this heterogeneity. The dense data sets generated in such experiments also provide useful opportunities for generation and validation of quantitative models of cell behaviour. In the review by Gerritsen and Pandit, 3 the modelling approaches are discussed in the context of CD8 T-cell state transitions from naive to effector and memory states.When attempting to investigate the process of clonal expansion of lymphocytes, where a small number of precursor cells proliferate into expanded clones, single-cell live imaging can provide good insights into cell numbers, proliferation, death and differentiation rates.In their review, Polonsky et al. 4 explore the possible applications of live imaging of single cells within microwell arrays and discuss their significant advances with respect to population analysis.One limitation of most of the single-cell technologies mentioned above lies in the lack of information about cell mor...