ISO 20391-1:2018 defines terms related to cell counting for biotechnology. It describes counting of cells in suspension (generally cell concentration) and cells adhered to a substrate (generally area density of cells). It provides key considerations for general counting methods (including total and differential counting, and direct and indirect counting) as well as for method selection, measurement process, and data analysis and reporting. ISO 20391-1:2018 is applicable to the counting of all cell types ? mammalian and non-mammalian (e.g. bacteria, yeast) cells. ISO 20391-1:2018 is not intended for counting of cells while in a tissue section or a biomaterial matrix. Several sector/application-specific international and national standards for cell counting currently exist. When applicable, the user can consult existing standards when operating within their scope (specific measurement techniques and/or applications).