Overview of standards. Standards are a set of guidelines, rules, or requirements, typically set by a regulatory body or product manufacturer. They ensure that either during manufacture of a product or in the final product, some aspect, such as safety, performance, or quality, is consistent. Standards are either written, in the form of guidance, standard operating procedures, and pharmacopoeia, or they are physical (material). Physical standards are characterized materials used as a benchmark against which to compare attributes of a product and ensure consistency. They can be agreed on across organizations and countries (consensus) or can be developer-specific and local (in-house) (i.e., the standards are only adopted within the organization in question). These in-house material standards, called RMs, are the focus of our review and can be further divided into a product RM, which is a representative batch of the product, or a method (measurement) RM, which provides a reference for analytical measurements. Abbreviation: RM, reference material.