| Both humanitarian and commercial considerations have spurred intensive search for methods to reduce the time and cost required to develop new therapies. The identification and use of surrogate endpoints is a general strategy that has stimulated much enthusiasm, but how can one establish the adequacy of a surrogate, in the sense that treatment effectiveness on the surrogate will accurately predict treatment effect on the intended, and more important, true outcome? What kind of evidence is needed, and what statistical methods portray that evidence most appropriately? The definition of validity, as well as formal sets of criteria, have been proposed, including use of the proportion explained, jointly the within-treatment partial association of true and surrogate responses, and the treatment effect on the surrogate relative to that on the true outcome. In a multi-centre setting, these quantities can be generalized to individual-level and trial-level measures of surrogacy. Consequently, a meta-analytic framework studying surrogacy at both the trial and individual-patient levels has been proposed. The framework commonly used will be sketched, also against the background of alternatives. A perspective will be given on further and ongoing developments.