Alzheimer's disease (AD) is characterized histopathologically by beta-amyloid-containing plaques, neurofibrillary tangles (NFT), reduced synaptic density, and neuronal loss in selected brain areas. Plaques consist of aggregates of a small peptide termed Abeta which is derived by proteolysis of the larger amyloid precursor protein APP, whereas NFT are composed of hyperphosphorylated forms of the microtubule-associated protein tau. Tau pathology in the presence of scant or no beta-amyloid plaques characterizes additional neurodegenerative disorders collectively called tauopathies. In the course of plaque and NFT formation, the major proteinaceous components of these lesions undergo post-translational modifications. In the case of tau, these include phosphorylation of mainly serine and threonine, but also tyrosine residues. In addition, tau is subject to ubiquitination, nitration, truncation, prolyl isomerization, association with heparan sulfate proteoglycan, glycosylation, glycation and modification by advanced glycation end-products (AGEs). This review aims to provide insight into the complexity of tau modifications in human tauopathies such as AD and frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Selected aspects of the post-translational modification of tau have been reproduced in transgenic animal models. Most of this work has been done in mice, but insight has also been gained from studies in the sea lamprey, the nematode C. elegans and Drosophila. Attempts have been made to link specific post-translational modifications with tau aggregation and nerve cell dysfunction.