Today, companies are more than ever dependent on a highly motivated workforce and hard-working employees. The purpose of this book is to expand the conceptual boundaries of work effort (WE) in order to gain critical insights into what makes people work hard. In spite of the acknowledged importance of WE, the concept was hitherto weakly understood, inconsistently defined, and lacked a clear conceptualization due to a lack of theoretical and empirical research. At theoretical level, this dissertation enhances the understanding of the WE concept, notably with respect to its antecedents. At practical level, the refined understanding will increase the leaders' ability to impact and manage their employees' WE levels.