In a brief historical description, it is shown that the prion model was developed from the biochemical and biophysical properties of the scrapie infectious agent. The biochemical properties of the prion protein which is the major, if not only, component of the prion are outlined in detail. PrP is a host-encoded protein which exists as PrP(C) (cellular) in the non-infected host, and as PrP(Sc) (scrapie) as the major component of the scrapie infectious agent. An overview of the purification techniques is given. Although chemically identical, the biophysical features of PrP(Sc) are drastically different in respect to solubility, structure, and stability; furthermore, specific lipids and a polyglucose scaffold were found in prions, whereas for nucleic acids their absence could be proven. The structure of recombinant PrP in solution is known from spectroscopic studies and with high resolution from NMR analysis. Structural models of PrP(Sc) were derived recently from electron microscopic analysis of two-dimensional crystals. Conformational transitions of PrP in vitro were studied with different techniques in order to mimic the natural PrP(C) to PrP(Sc) conversion. Spontaneous transitions can be induced by solvent changes, but at present infectivity cannot be induced in vitro.