Work engagement is the 'harnessing of organization member's selves to their work roles: in engagement, people employ and express themselves physically, cognitively, emotionally and mentally during role performances'.:694 Three aspects of work motivation are cognitive, emotional and physical engagement. Work engagement is the 'harnessing of organization member's selves to their work roles: in engagement, people employ and express themselves physically, cognitively, emotionally and mentally during role performances'.:694 Three aspects of work motivation are cognitive, emotional and physical engagement. There are two schools of thought with regard to the definition of work engagement. On the one hand Maslach and Leiter assume that a continuum exists with burnout and engagement as two opposite poles. The second school of thought operationalizes engagement in its own right as the positive antithesis of burnout. According to this approach, work engagement is defined as a positive, fulfilling, work-related state of mind that is characterized by vigor, dedication, and absorption. Vigor is characterized by high levels of energy and mental resilience while working, the willingness to invest effort in one's work, and persistence even in the face of difficulties; dedication by being strongly involved in one's work, and experiencing a sense of significance, enthusiasm, inspiration, pride, and challenge; and absorption by being fully concentrated and happily engrossed in one's work, whereby time passes quickly and one has difficulties with detaching oneself from work. Organizations need energetic and dedicated employees: people who are engaged with their work. These organizations expect proactivity, initiative and responsibility for personal development from their employees. Work engagement as measured by the UWES is positively related with, but can nevertheless be differentiated from, similar constructs such as job involvement and organizational commitment, in-role and extra-role behavior; personal initiative, Type A, and workaholism. Moreover, engaged workers are characterized by low levels of burnout, as well as by low levels of neuroticism and high levels of extraversion. Also they enjoy good mental and physical health. Generally, work engagement is conceptualized as a relatively stable phenomenon. This can be explained by the presence of specific job and organizational characteristics. However, engagement is not a fixed state: the level of engagement can even fluctuate on a weekly or day-to-day basis. Increasingly, attention is being paid to these short-term fluctuations by conducting experience sampling studies and diary studies.Christian, Garza, and Slaughter (2011) meta-analyzed over 90 engagement research studies. They found that engagement is distinct from job satisfaction, organizational commitment, and job involvement. The three aspects of work engagement (vigor, dedication and absorption) are assessed by the Utrecht Work Engagement Scale (UWES), which is currently available in 20 languages and can be used freely for non-commercial purposes. In addition a short form and a student version are available. The reliability and validity of the UWES is documented is various studies. The Oldenburg Burnout Inventory (OLBI) is an alternative instrument for the assessment of work engagement. It consists of two dimensions: exhaustion-vigor and cynicism-dedication. The recently operationalized construct of drive was proposed as the key differentiating individual difference dimension of work engagement and has shown promising psychometric results. Measured on the basis of 13 facets and three aspects (passion, effort, ideation), drive is argued to predict future motivation and engagement levels better than measuring these attributes directly at any point in time, in any given context.