Iso-α-acids are quantitatively the most important hop-derived fraction in beer. In addition to CO2 and ethanol, iso-α-acids are considered the primary beer flavour components. They impart the typical bitter taste and depending on the desired beer bitterness, their concentration varies from 10 up to 100 mg/L, according to the beer type. This article is to give a review on the knowledge about the formation of iso-α-acids and the mechanisms of their degradation in the beer matrix during storage. Iso-α-acids arise from the isomerisation of hop α-acids during wort boiling. In addition to the isomerisation reaction, the beer’s bitter acids degradation has extensively been studied. Iso-α-acids are subjected to both oxidative and nonoxidative transformations. As a result, both the intensity and the quality of the beer bitterness are adversely affected. Both trans- and cis-iso-α-acids are susceptible to degradation in the presence of reactive oxygen species and light. Stored in the dark, trans-iso-α-acids in beer were found to be markedly more unstable than their cis-counterparts, and a harshy, lingering bitter taste simultaneously develops. This flavour defect occurring during beer ageing is attributed to the proton-catalysed cyclisation of trans-iso-α-acids into tri- and tetracyclic degradation products. These non-volatile degradation products of trans-iso-α-acids have only recently been identified. In contrast, only few reports deal with the formation of volatiles from iso-α-acids upon beer ageing. The relation between bitter acids degradation and aldehyde formation is cited in this review.