Alzheimer’s disease (AD) is characterized by two pathological hallmarks, namely, senile plaques and neurofibrillary tangles (NFTs). The former are mainly composed of amyloid-β peptides (Aβ) while the latter consists mainly of filaments of hyperphosphorylated tau. Cyclin-dependent kinase 5 (cdk5) has been implicated not only in the tangle pathology, but recent data also implicate cdk5 in the generation of Aβ peptides. Since both Aβ peptides and NFTs are believed to play a role in neurodegeneration in AD, this proline-directed serine/threonine protein kinase is likely to contribute to the pathogenesis of AD. In vitro and in vivo animal data demonstrate the ability of cdk5 to induce phosphorylation and aggregation of tau, and NFT deposition and neurodegeneration. Findings from AD brain samples also show an elevated cdk5 activity and conditions that support the activation of cdk5. Evidence for the role of cdk5 in regulating Aβ production is just emerging. The mechanisms for this potentially damaging activity of cdk5 are largely unknown although amyloid precursor protein and presenilin-1 are both cdk5 substrates.