The escalating interest in research on interventions that may affect aging processes has necessarily focussed attention on the outcome measures. The desirable characteristics of these "biomarker variables" have been widely discussed. This article offers some reflections on validity, reliability, and generalizability of biomarkers. It is argued that our comprehension of aging will evolve iteratively from application of a diversity of biomarker variables. Each of these will have strengths and shortcomings from methodological and measurement points of view. The siren song that a "gold standard" index of aging can be found should be ignored.