Traditional aging protocols typically examine only the e↵ects of a limited number of stresses, and relatively harsh conditions may trigger degradation mechanisms that are not observed in actual situations. Environmental aging is, in essence, the complex interaction of multiple mechanical, physicochemical and biological stresses. As yet, there is no (pre)standardized procedure that addresses this issue in a satisfactory manner. Mesocosm experiments can be designed to specifically cover the aging of nanomaterials while characterizing the associated exposure and hazard. The scenario of exposure and the life time of the nanomaterial appear as the predominant factors in the design of the experiment, and appropriate precautions need to be taken. This should the subject of guidance that may be divided into product/application categories. 1. The Importance of the Aging Factor There are still major knowledge gaps in the risk assessment of nanomaterials, especially in the post-production stages of their life cycle. However, in most cases, the use and end-of-life phases are by far the longest periods, and this extended duration is a potential concern with regard to exposure to nanomaterials, as well as associated hazards. Apart from a few exceptions, nanomaterials with the desired properties are not used "as is", but are embedded in the final product after several formulation steps (e.g., encapsulation, attachment/embedding into a matrix, etc.) for targeted functionality, better product safety, or simply ease of use. As a consequence, the pristine/isolated nanoparticle is no longer the only compound that needs to be considered in risk assessment. The nature and the structure of the bearing matrix control the exposure to the nanomaterial and the associated hazard. In other words, the entire nano-enabled product and its evolution with time, i.e., aging, become determining factors in the risk assessment. While a newly manufactured product should not be a concern for the consumer or the environment, aging is likely to change the mobility and the speciation of embedded nanomaterials, and may cause potential adverse e↵ects towards the consumer and/or the environment. As a consequence, attention needs to be paid to the degradation of the matrix, shell(s) and coating(s) surrounding the nanomaterial in order to avoid its release in a form that is, or might become, hazardous. It is noteworthy that understanding the (bio)physicochemical mechanisms of matrix degradation leading to nanomaterial release is challenging and requires the use of powerful characterization tools [1-3]. Therefore, even if release is a direct consequence of the degradation of the matrix, aging is often studied via the quantification of the release with simple metrics.