Diverse human disorders, including most neurodegenerative diseases, are thought to arise from the misfolding and aggregation of an underlying protein. We have recently described a novel technology to amplify cyclically the misfolding and aggregation process in vitro. This procedure, named protein misfolding cyclic amplification (PMCA), conceptually analogous to DNA amplification by PCR, has tremendous implications for research and diagnosis. The PMCA concept has been proved on the amplification of prions implicated in the pathogenesis of transmissible spongiform encephalopathies (TSE). In these diseases, there is a tremendous need for early and sensitive biochemical diagnosis to minimize the further spreading of the prion infectious agent through the food chain. In this chapter, we describe the principles behind the PMCA technology, its application, and methodology to detect minute quantities of misfolded prion protein and its potential to be used for amplification of misfolding of other proteins implicated in diverse diseases.