Processed foods are typically praised/revered for their convenience, palatability, and novelty; however, their healthfulness has increasingly come under scrutiny. Classification systems that categorise foods according to their “level of processing” have been used to predict diet quality and health outcomes and inform dietary guidelines and product development. However, the classification criteria used are ambiguous, inconsistent and often give less weight to existing scientific evidence on nutrition and food processing effects; critical analysis of these criteria creates conflict amongst researchers. We examine the underlying basis of food classification systems and provide a critical analysis of their purpose, scientific basis, and distinguishing features by thematic analysis of the category definitions. These classification systems were mostly created to study the relationship between industrial products and health. There is no consensus on what factors determine the level of food processing. We identified four defining themes underlying the classification systems: 1. Extent of change (from natural state); 2. Nature of change (properties, adding ingredients); 3. Place of processing (where/by whom); and 4. Purpose of processing (why, essential/cosmetic). The classification systems embody socio-cultural elements and subjective terms, including home cooking and naturalness. Hence, “processing” is a chaotic conception, not only concerned with technical processes. Most classification systems do not include quantitative measures but, instead, imply correlation between “processing” and nutrition. The concept of “whole food” and the role of the food matrix in relation to healthy diets needs further clarification; the risk assessment/management of food additives also needs debate. •Classifications were created to link processed foods to nutrition and health outcomes.•There is no consensus on what determines the level of food processing.•Classification includes the extent, nature of change, place and purpose of processing.•Processed food concepts relate to naturalness, additives, convenience, home cooking.•Food classifications embody social and cultural elements and subjective terms.