Alzheimer's disease (AD) is the most common form of dementia. In a small number of cases AD is genetically inherited. Mutations are associated with so far three genes. These genes encode the beta-amyloid precursor protein (beta APP), as well as presenilin (PS) 1 and -2. Mutations in all three genes affect the generation of amyloid beta-peptide (A beta), which is the major component of senile plaques. Mutations in the PS genes occur much more frequently as those associated with the beta APP gene and can cause the earliest onset of AD ever recorded. PS genes are not only involved in familial AD but also play a functional role in the general production of A beta. Therefore PS proteins are key molecules, which will allow us to understand fundamental aspects of the molecular mechanisms involved in AD. Here we will summarize the pathological as well as biological function of PS and demonstrate that simple systems, such as cultured cells and the worm Caenorhabditis elegans can be used for modern AD research.