This review synthesizes conceptual and empirical research on the emergence of individual and collective leadership in task groups, and proposes avenues for leadership research. To advance multilevel study of leadership emergence, including emergence of distributed and shared leadership, the paper reviews research on individual leader emergence, structured around two identified theoretical mechanisms—one of leadership achievement (i.e., based on functional behaviors) and another of leadership ascription (i.e., based on nominal characteristics). These approaches compete to elucidate individual leader emergence in task groups as influenced by individual traits, states, and behaviors. However, current approaches to leadership in groups rely on functional achievement explanations of how collective leadership emerges, influenced by positive states and behaviors. Attention to ascription–achievement mechanisms in leadership emergence is warranted at the collective level of analysis.