Conformational diseases are diseases where cellular functions are compromised because of misfolded proteins. The conceptional framework of conformational diseases is found in the cellular protein quality control systems which in the normal and young cell eliminate misfolded proteins. Many inherited genetic defects result in the misfolding of proteins, which may lead to recessive disorders if the proteins in question are totally or partly eliminated or to dominant diseases if the proteins slip through the protein quality control and accumulate in the cell. These inherited diseases are all early onset. Misfolding may also occur in proteins with an intrinsic ability to aggregate and in oxidatively damaged proteins, which accumulate by ageing. If the protein quality control systems are not sufficiently efficient cell toxic protein complexes may accumulate. This pathogenesis is a major contributing factor in the development of late onset neurodegenerative disorders.