Introduction Leadership is an interpersonal, person-oriented, social influence (Endres & Weibler, 2016), which guides in direction, course, action, and opinion (Bennis & Nanus, 1985). To that end, it is different from management, which is task oriented in the sense of bringing about or accomplishing something, being responsible for, or conducting something (Bennis & Nanus, 1985). Leadership requires a leader, but leaders do not operate in a vacuum, nor do they need to be formal managers. For a long time, the terms “leader,” “leadership” and “manager” were used almost interchangeably, and only recently has there been a distinction in their use (Alvesson & Sveningsson, 2003; Crevani et al., 2010). The development of leadership research over the years has also focused on different aspects of leadership, ranging from an interest in the traits, characteristics, and competencies of individual leaders, to an interest in how leadership is practiced in social settings by leaders, co-leaders, and followers in interaction (Carroll et al., 2008). Recent years have brought much insight about the importance of leadership as a complement to management in projects and has added a variety of perspectives to leadership. Examples include the traditional view of project and program managers as leaders and their associated leadership styles. This person-centric perspective has addressed the particular leadership styles of these roles (e.g., Keegan & Den Hartog, 2004) as well as personal characteristics that bring about certain leadership styles (e.g., Dulewicz & Higgs, 2005). We call this the vertical leader and his or her leadership style. Other research has looked at leadership that emerges from teams or individuals in a team and complements the leadership of the vertical leader. Examples of this include the studies on shared leadership (Pearce & Conger, 2003; Crevani et al., 2007) and its related processes (e.g., Cox, Pearce, & Perry, 2003). We call this approach the horizontal leader and his, her or their leadership style. Most recently, researchers have started to investigate the balance and situational contingency of vertical and horizontal leadership in projects. They showed the particular circumstances under which vertical leaders in projects make way for horizontal leaders to temporarily partake in leading the project (e.g., Müller et al., 2016). We call this balanced leadership, i.e., a situation in which the balance between vertical and horizontal leadership is appropriate.