Personalized medicine introduces the promise to use molecular markers that signal the risk of disease or its presence before clinical signs and symptoms appear. This information underlies a new healthcare strategy focused on prevention and early intervention, rather than reaction to advanced stages of disease. Such a strategy can delay disease onset or minimize symptom severity. The molecular foundations that enable personalized medicine include detection of variation in nucleotide sequence of genes and in characteristic patterns of gene expression, proteins and metabolites. Genetic and molecular patterns are correlated with disease manifestations, drug responses, treatment prognosis, or prediction of predisposition to future disease states. However, the uncertainties for personalized medicine are considerable, including economic, ethical, legal, and societal questions. Although much of its promise remains unproven to date, the foundations of personalized medicine appear solid and evidence is accumulating rapidly pointing to its growing importance in healthcare (Fig. 1).