It is widely accepted that project leaders should adapt their behaviour to meet the unique leadership demands of a variety of situations. Recently, vertical, shared, and horizontal styles of leadership have gained prominence, especially in the project management literature. Several factors are believed to play a role in determining an appropriate balance between these leadership styles. This theoretical study explores the influence of project types, the stage in the project life cycle, organisational project management maturity, and the level of trust and collaboration between project team members on the appropriate balance of leadership styles in projects. This paper presents a conceptual framework of these factors, while empirical results will be reported on in the sequel to this paper.