Approximately 65 000 people in Norway are suffering from dementia and the prevalence will increase as the population continues to age. The importance of biomarkers of Alzheimer’s disease and other dementias is increasing as well. There is a need for biomarkers to aid the diagnosis and to evaluate progression and the effect of therapeutic strategies. A biomarker may be a genetic trait, a change in structural or functional feature or a biochemical change, such as a protein or metabolite. The purpose of this paper is to review the currently available data on biomarkers assessed in plasma and cerebrospinal fluid in dementias, focusing on Alzheimer’s disease. The most extensively evaluated biomarkers are amyloid-β-proteins and total and phosphorylated tau protein in the cerebrospinal fluid. These markers are shown to differentiate between Alzheimer’s disease and several important differential diagnoses, including normal aging, depression and chronic neurological diseases like Parkinson’s disease. However, the specificity against other dementias is lower. Measures related to lipoprotein metabolism, oxidative stress and inflammation also appear altered in AD relative to controls, but lack sufficient discriminatory power. New proteomic technologies are likely to play an increasing role in identifying potential biomarkers of cognitive decline and dementia syndromes.