Cytometry Part A, the journal of quantitative single-cell analysis, publishes research and reviews on innovative studies employing single-cell measurement, manipulation, and modeling techniques. We explore molecular and cellular functional mechanisms obtained by cytometry techniques, with a particular focus on cellular heterogeneity, rare cell subpopulations, understanding cellular functionality, tracing lineages of cellular phenotypes, and other emerging topics of single-cell biology. The official journal of the International Society for Advancement of Cytometry.