 The search for reliable biological age indicators has been ongoing for over three decades without success until recently, with advances in molecular biology increasing potential candidate biomarkers. This review summarizes current state-of-the-art findings on six types of biological age predictors, epigenetic clocks, telomere length, transcriptomic predictors, proteomic predictors, metabolomics-based predictors, and composite biomarker predictors. Promising developments consider multiple combinations of these various types of predictors, with the epigenetic clock being the most promising new biological age predictor, but its true value as a biomarker of aging requires longitudinal confirmation. This article was authored by Julia Jilhova, Nancy L. Peterson, and Sarah Hagg.