Educational effectiveness research separates hypothetical causes of performance differences into "given", "contextual", "endogenous", or simply "prior" conditions, on the one hand, and malleable factors, or treatments, on the other hand. Recent studies indicate that the effects of background conditions tend to be bigger, and those of malleable variables and interventions smaller, than usually expected. These findings give reason to pose "limited malleability" as the central hypothesis of the special issue. This hypothesis is addressed in the 5 articles that make up this special issue. The themes addressed in these articles are respectively: optimizing the choice of adjustment variables, the development of a nomological network of educational achievement at country level, the stability of system-level educational performance, modelling approaches to the estimation of size, stability, and consistency of school effects, and treatment effects in schooling. The final article makes up the balance on the "limited malleability" thesis and discusses implications for educational policy and practice.