In 1986, Robert Coffman and Timothy Mossman first described the division of CD4⁺ T cells into functional subsets, termed T helper 1 (T_H1) and T_H2, based on cytokine production, and in doing so unwittingly opened a Pandora's box of complexity and controversy. Although the mechanisms that regulate T_H1 and T_H2 cells are now well known, recent descriptions of other CD4⁺ T cell subsets — such as regulatory T cells, T follicular helper cells, T_H17, T_H22 and most recently T_H9 and T_H22 cells — have questioned how we think of T cell subsets and what commitment to a functional T cell subset means. Here, Nature Reviews Immunology asks four leaders in the field their thoughts on the functional plasticity of T cell subsets.