Over the last few decades, an increasing body of literature has focused on understanding the unique role line managers play in the modern organisation. Line managers are responsible for directly managing individual employees or teams, and in turn, report to a higher level of management on the performance and well-being of the employees or teams they manage. Irrespective of their integral role within the organisation, research remains limited in understanding "how" line managers navigate the unique context of their work environment and take part in organisational HR practices. Without a clear understanding of the "how" we are limited in our ability to differentiate between the pressures line managers face as both "the managers" and "the managed". Person-environment fit theory provides a unique lens to understand this missing area of line manager research. Our chapter reviews the extant literature on line managers and person-environment fit theory to explore the history, current conceptualisations, methodological considerations, and recent advancements to illustrate the theoretical relevance of P-E fit theory in examining line manager fit within the organisational environment. Our chapter also provides researchers with several suggestions for future research that can be used to advance our understanding of line managers.