Polyploidy is defined as an increase in genome DNA content. Throughout the plant and animal kingdoms specific cell types become polyploid as part of their differentiation programs. When this occurs in subsets of tissues within an organism it is termed somatic polyploidy, because it is distinct from the increase in ploidy that is inherited through the germline and present in every cell type of the organism. Germline polyploidy is common in plants and occurs in some animals, such as amphibians, but will not be discussed further here. Somatic polyploid cells can be mononucleate or multinucleate, and the replicated sister chromatids can remain attached and aligned, producing polytene chromosomes, or they can be dispersed (Figure 1). In this Primer, we focus on why somatic polyploidy occurs and how cells become polyploid — the first of these issues being more speculative, given the status of the field.