We characterize the entire functions ℓfor which the induced nonlinear superposition operator f →ℓof one Besov space Bp into another Bq, where B∞ can be taken to be any of the following natural spaces: VMOA, BMOA, B0, and B. We do the same for the superpositions from one unweighted Dirichlet-type space Dp into another, and from Bp into the weighted space Dqά. The admissible functions are typically polynomials whose degree depends on p and q, or entire functions whose order and type are determined by those exponents. We prove some new Trudingertype inequalities for analytic functions along the way.