Epoxide hydrolases (EHs) metabolize highly reactive epoxides with mutagenic and carcinogenic potential to the less reactive corresponding diols and are therefore traditionally viewed as detoxicating enzymes. In few instances, however, the diols themselves can be precursors of further reactive metabolites, and EH may thereby contribute to metabolic toxification. The most important xenobiotic-metabolizing EH is the microsomal EH (mEH). Furthermore, evidence is emerging that mammalian EHs fulfill roles other than detoxication. Meanwhile, the role of soluble EH (sEH), whose physiological substrates are epoxyeicosanoids (epoxyeicosatrienoic acids, EETs), signaling molecules involved in a broad variety of regulatory pathways, is well documented. sEH is therefore considered a new drug target as respective inhibitors promise therapeutic potential in the treatment of hypertension, pain, and possibly other diseases. Two new human EHs, EH3 and EH4, have been recently identified. Their expression pattern and substrate specificity strongly suggest a role in physiological processes, similar to sEH, rather than in metabolizing xenobiotic compounds.