A key event in the pathogenesis of prion diseases is the conversion of the normal cellular isoform of the prion protein into the disease-associated isoform, but the mechanisms operating in this critical event are not yet fully understood. A number of novel approaches have recently been developed to study factors influencing this process. One of these, the protein misfolding cyclical amplification (PMCA) technique, has been used to explore defined factors influencing the conversion of cellular prion protein in a cell-free model system. Although initially developed in animal models, this technique has been increasingly applied to human prion diseases. Recent studies have focused on the role of different isoforms of the disease-associated human prion protein and the effects of the naturally occurring polymorphism at codon 129 in the human prion protein gene on the conversion process, improving our understanding of the interaction between host and agent factors that influence the wide range of phenotypes in human prion diseases. This technique also allows a greatly enhanced sensitivity of detection of disease-associated prion protein in human tissues and fluids, which is potentially applicable to disease screening, particularly for variant Creutzfeldt-Jakob disease. The PMCA technique can also be used to model human susceptibility to a range of prions of non-human origin, which is likely to prove of considerable future interest as more novel and potentially pathogenic prion diseases are identified in animal species that form part of the human food chain.