Alzheimer's disease is a degenerative disorder of the human central nervous system that results in a progressive loss of memory and intellectual abilities. It is strongly related to aging and it is thus assumed that 0.5% of the total population, and up to 30% of eighty-year-olds suffer from the disease. Many require expensive institutional care, often over long periods, as there is no effective treatment at present. Abundant amyloid plaques and neurofibrillary tangles constitute the two major neuropathological lesions that characterize the brains of patients with Alzheimer's disease. The amyloid plaque consists mainly of a soluble polypeptide of 42-43 amino acids called beta-amyloid. beta-Amyloids is derived by an alternative cleavage of the much larger amyloid precursor protein (APP), but it is not known which proteolytic enzyme is responsible for this alternative cleavage. In contrast to plaques, the neurofibrillary tangles are formed intracellularly and the number of them seems to correlate with the progression of the disease. Their main components are paired helical filaments (PHF) which seem to consist almost entirely of the protein tau. The normal function of tau is to bind to microtubules and thereby stabilize the nerve cell's structure and integrity. In contrast to normal tau, PHF-tau is heavily phosphorylated, and it is assumed that this phosphorylation is the underlying cause of the formation of PHF and the neurofibrillary tangles. Despite extensive research it is still not known which enzymes are responsible for the over phosphorylation of tau that occurs in Alzheimer's disease. If they could be identified and controlled pharmacologically, an effective treatment of the disease might be possible.