Frijda has devoted his career to human emotions. In his early career he focused on facial expressions, which was an acceptable approach in the then dominant behaviourist tradition in which emotions were considered epiphenomena. Frijda developed a multifocal theory of emotions with the concept of "action tendency" as focal issues. Emotions are, in this view, tendencies to engage in behaviour influenced by the needs of the person. This theory was outlined in detail in The Emotions (1986), his magnum opus which was first published in English and two years later in a Dutch translation.