The existence of individual variations in behaviour has been demonstrated in all species investigated. Horses do show individual variations in a variety of behaviours from an early age on, as well as in behavioural traits underlying temperament/personality. Temperament is generally described as the “biological” (i.e.genetic) basis on which personality, as a result of environmental influences (i.e. experience), will be constructed. Indeed, genetic factors such as breed or patriline do have an influence on aspects like emotional reactions to fear-inducing situations or cognitive abilities while environmental factors, such as the conditions of life or the type of work performed, have also an influence on horse personality. We will review here the existing evidence of this interplay between genetic and environmental influences, showing that different traits can be differently influenced, and then look more thoroughly at the processes involved. A particular accent will be given to the influence of the human-horse relation on personality traits of horses, both at adult stages through management, daily interactions and type of work, and at early stages through management and handling of foals. The question of the stability of traits across situations and over time, that is of the possible predictibility of behavioural traits, is central, especially where the transfer from experimental situations (behavioural tests) to working situations and performance is concerned. The question of methodological approaches (behavioural tests, observations, questionnaires) can be crucial here and will be discussed. In overall, this review should help understanding the determinants of horse temperament/personality traits and their implication in terms of choice of the appropriate horse for a given owner, better management and training for a better welfare and horse-human relationship. Genetic influences may lead to different susceptibilities to the same environmental conditions which mean that there are no such thing as “bad” or “good” horse temperaments but rather different horse types that may require different management or training schedules.