Ketone body utilisation is of special importance in times of fasting/starvation or increased energy demand. However, both formation and utilisation of ketone bodies (ketogenesis and ketolysis) can be impeded by inborn errors of metabolism. In case of genetic deficiency of mitochondrial 3-hydroxy-3-methylglutaryl-coenzyme A synthase (mHMGS) or of 3-hydroxy-3-methylglutaryl-coenzyme A lyase (HMGL), the formation of ketone bodies is impaired. If one of the enzymes of ketolysis is affected, namely, succinyl-CoA:3-oxoacid CoA transferase (SCOT) or methylacetoacetyl-CoA thiolase (MAT, “β-ketothiolase”), ketones accumulate and a life-threatening ketoacidosis may result. Since treatment options allow to minimise the risk for metabolic decompensations, awareness of those diseases is important, as is information on how to treat and to prevent clinical manifestations.