Two accounts of welfare that are currently used in animal welfare assessments are firstly the Brambell committee's account that focuses on the absence of negative feelings and the capacity to display normal behaviour, and secondly an alternative account that focuses on the animal's capacity to display its normal behaviour and thereby to adapt to changing living conditions up to a level that it experiences as positive. Both accounts seem to be concerned with both, the animal's state of mind and its capacity to behave according to its nature. In order to better understand - and maybe even improve - these accounts of welfare, it is helpful to ask two questions: Firstly, which states of mind are important with regard to welfare? Secondly, how does the importance of those states of mind relate to the importance of behaviour in accordance with the animal's nature? Daniel Haybron's theory of welfare provides new and original answers to both questions. Haybron's affective state view conceives of happiness as a 'mental state', but unlike hedonism its focus is on emotions and moods, rather than merely on (un)pleasant experiences. The affective state view is concerned with a being's overall emotional state and includes a being's propensity for experiencing various emotions and moods. According to Haybron self-fulfillment is central to welfare, and an animal's emotional state is part of its self-fulfillment. Haybron's aswers might be useful for understanding and even improving the accounts that are currently used in assessments of animal welfare. In order to find out whether this is indeed the case, Haybron's theory of welfare and its implications for animal welfare assessment should be further explored and evaluated.