Biomarkers are characteristics that are objectively measured and evaluated as indicators of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention. Amyloid measures become abnormal early in the Alzheimer's disease process and have only weak correlations with disease progression and cognitive decline; cerebrospinal fluid tau, brain atrophy, and reduced cortical metabolism correlate with cognitive measures and disease progression. Combinations of biomarkers have higher correlations and are better predictors of future decline than single biomarkers. Current biomarkers do not account for all of the variance in Alzheimer's disease; a more complete repertoire of biomarkers that more comprehensively assay the disease process is needed.