Prions, the causative agents of prion diseases, consist of the abnormal isoform of prion protein, PrP(Sc). PrP(Sc) is generated by conformational conversion of the normal isoform of prion protein, PrPc, a glycosyl-phosphatidyl-inositol-anchored glycoprotein abundantly expressed on the surface of neurons. Prions or PrP(Sc) having invaded the body interact with PrP(c) and induce changes in structure of the interacting PrP(c) into that of PrP(Sc), leading to prion replication. At the same time, this constitutive conversion causes the detrimental accumulation of PrP(Sc) in the brain tissue. Here, I will introduce the structural and biochemical properties of PrP(c) and PrP(Sc) and discuss the nature of prions and the mechanisms of prion replication in more detail.