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The main object of this report is to review and summarize the current state of the scientific literature relating to physico-chemical ageing of ethylene-norbornene copolymers (ENCs). These processes occur through different means such as photodegradation under the effect of ultraviolet radiation, temperature and ionizing radiation (under air or oxygen). In most cases ageing causes chains scissions that lead to the creation of degradation compounds that exhibit high mobility, corresponding to their low molecular weight and/or high polarity. Cross-linking can also occur, leading to the modification of mechanical properties. The presence of oxygen during ageing is of great importance for the physico-chemical modifications of those polymers in terms of oxidation and thermal stability. ENCs belong to cyclic olefin copolymers (COC) class, whose synthesis was developed in the late 1950s. Nevertheless, commercial applications are relatively new. They attract growing interest in different fields, due to their very useful physical and chemical properties such as transparency, heat and chemical resistance, low permeability to gas as well as biocompatibility. In the literature, a large number of papers deal with the synthesis and characterization of different grades of ENC, but only a few deal with the effects of ageing on these materials. This literature review consists of a summary of the main milestones in the development of COC as well as of cyclic olefin polymers (COP), a review of main techniques for predicting ageing, degradation of additives and interaction between COC/COP and contact environment as well as toxicity of the extractables.