Traditional methods of project management are not appropriate for complex projects anymore. Since projects are becoming increasingly complex and uncertain, interaction between activities and resources is growing in ways not considered by these methods. Nowadays, managers need more agile project management methods that are able to recognize and deal with uncertainty and to produce the expected results. Lean project management, the most recent approach of lean methodology, appears as an alternative approach capable of dealing with complexity and uncertainty. The latest investigations in the field show that traditional methods are still adequate for simple projects, while lean methods are more appropriate for complex projects. This thesis aims to investigate the nature of lean project management and to examine project risk management processes so that managers can assess the complexity of projects before their beginning and decide which method to apply in order to manage them.