Surrogate endpoints are useful when they can be measured earlier, more conveniently, or more frequently than a true endpoint. Regulatory agencies are introducing provisions and policies relating to the use of surrogate endpoints in registration studies. 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? 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-center setting, one distinguishes between individual- and trial-level surrogacy. Consequently, a meta-analytic framework studying surrogacy at both the trial and individual-patient levels has been proposed, for various settings and endpoint types. The framework will be sketched, also against the background of alternatives, with particular attention to causal-inference, and paying attention to the use of information theory.