The concept for categorization of eating episodes in dietary surveys was originally developed in studies of shift workers to compare 'meal patterns' between night and day work shifts. The concept has been further improved through experience from applications in dietary surveys in other populations. In this paper, results from categorization of eating episodes in shift workers, elderly women and men during life transition periods, elderly female leg ulcer patients and obese men and their lean controls are shown and discussed. The categorization concept is based on seven food categories with food items of similar nutrient characteristics within each category. Each eating event is categorized as any of four types of 'meals' or four types of 'snacks' due to its combination of food categories. Thus, categorization is based on visible properties (food types) but at the same time reflecting invisible properties (nutrients). Criteria is also established to sub-categorize the 'meal' types as being either 'prepared' or 'quick-prepared' from a behavioural perspective. Use of a defined and reliable concept for categorization is necessary to study eating episodes in dietary surveys, their determinants and also consequences on health and performance. Nocturnal eating during the circadian nadir might affect nutritional status. Since increasingly western populations appear to be moving from regular and planned meals to more episodic eating 'around the clock', such analyses are of increasing interest in a bio-social perspective.