Project planning is widely thought to be an important contributor to project success. However, does the research affirm its impact and give guidance as to how much effort should be spent planning? The literature in project management, and to a lesser extent in general management, is reviewed to find the reported link between planning and project success. Overall, the literature points to a strong link between planning and project success. A summary of the available studies shows unexpectedly consistent empirical results for the correlation of planning quality and success. The literature appears to be generally consistent showing an average value of R2 = .33 correlation with efficiency and R2 = .34 for overall project success. This indicates a significant impact if compared to the reported approximate 20-33% recommended planning effort.