Abstract: Certain features of a project may indicate proper management actions to complete it successfully. Complexity can be an example dealing with these features. The present study aimed at reviewing the available research on project complexity. Researches carried out during 1996-2015 on such areas of complexity as primary definitions, dimensions, etc. were considered. Available researches were studied and categorized according to their subjects and settings. This categorizing allows us to know which research on what areas has been carried out. Thus, through studying their approaches and points of view, it would be possible to address the faults and gaps for future research or project complexity.