The number of national, institutional, and subject repositories of grey literature has increased dramatically over recent years. The Directory of Open Access Repositories (OpenDOAR) currently lists 5848 repositories, a 75% increase over the last five years. Most of these repositories hold grey literature of one type or another. The degree of development of these repositories is mixed, some are of questionable quality while others are exemplars, so it is useful to define what constitutes a well-developed repository. A well-developed repository can be seen as one that meets the needs of end users, as well as the interests of authors and sponsoring organizations. Characteristics, such as timeliness, openness, user-friendliness, accuracy, and completeness, are proposed as those which meet user and institutional needs and define the degree of development for a given repository. Timeliness refers to the speed at which materials in the scope of the repository are made available to the public. Openness is the degree to which material is accessible as well as shareable. User-friendliness is a subjective quality but is defined by the ease of use of the repository’s user interface. Accuracy can be measured in many ways – the verisimilitude of metadata, the suitability of indexes and search results, the percentage of dead links to external resources such as full text, and other measures. Finally, completeness describes how well a repository encompasses its scope. The International Nuclear Information System (INIS) has been in operation since 1970 as a repository for grey and traditional literature in all areas of nuclear science and technology. It existed before the wide adoption of information management principles, and invented methods and workflows to fulfil its mission. Therefore, there are gaps between the ideal repository, embodied in the outlined characteristics, and the repository as it currently stands. These gaps are identified and solutions, as well as a plan for implementation, are proposed.