Recent research provides evidence that, contrary to implicit assumptions in much of the strategic human resource management (SHRM) literature, human resource (HR) systems and practices are in fact enacted with substantial variation across units even within organizations, with such variation largely a function of the line managers involved in implementing HR practices in the units under their supervision. While instrumental in demonstrating the critical role that line managers play in facilitating the causal chain linking organizations’ HR practices with intended employee and organizational outcomes, we contend that the focus of this research on HR practice implementation as a singular and unidimensional characterization of line managers’ involvement in human resource management (HRM) represents an oversimplification on several counts. Broadly, we propose that this focus fails to account for the varied nature of line managers’ downward influences in the context of HRM. Thus, we integrate insights from research on HR practice implementation, workforce differentiation, and autonomous strategic behavior to develop a more complete understanding of line managers’ downward involvement in HRM. Based on our synthesis of relevant insights from these literatures, we propose a research agenda focusing on questions spanning four broad areas with the aim of fostering and guiding future SHRM scholarship to further our understanding of the antecedents, processes, and consequences associated with line managers’ influences on HR system content and process in organizations.