The peptidyl prolyl cis/trans isomerase Pin1 specifically binds phosphorylated Ser/Thr-Pro protein motifs and catalyzes the cis/trans isomerization of the peptide bond. Accumulating studies have revealed that Pin1 isomerase activity is regulated by its post-translational modifications, including phosphorylation and oxidation. Various transcription factors and regulators have been identified as substrates for Pin1. It enhances AP-1 activity via isomerization of both c-Jun and c-Fos for cellular proliferation and stabilizes the oncosuppressors p53 and p73 against DNA damage at the checkpoint. We demonstrated the association between the intracellular form of Notch1 (NIC) and Pin1 by analyzing Pin1/p53 double-knockout mice. Pin1 also regulates the post-transcriptional level of some cytokines, associated with asthma, that possess 3' untranslated region AU-rich elements (AREs) via interaction withAUF1, the nucleoprotein in the ARE-binding complex. Pin1 has been identified as the molecular partner of tau and amyloid precursor protein (APP), the key factors of Alzheimer's disease (AD). It interacts with the phosphorylated Thr-231 of tau and regulates its activity to bind microtubules. It further interacts with the phosphorylated Thr-668 of APP and affects its metabolism. Thus, Pin1 is probably involved in the pathogenesis of human diseases, including cancer, asthma, and AD, presenting an attractive target for future therapeutical drugs.