Abstract. We characterize stability under composition of ultradifferentiable classes defined by weight sequences M , by weight functions ω, and, more generally, by weight matrices M, and investigate continuity of composition (g, f ) → f • g. In addition, we represent the Beurling space E (ω) and the Roumieu space E {ω} as intersection and union of spaces E (M ) and E {M } for associated weight sequences, respectively.