The chapter provides guidance pertaining to the development of correlations for the adjustments needed to account for unknown and/or underappreciated (U/U) risk and for the uncertainty in each correlation. It does this by outlining the fundamentals of accounting for U/U risk as a contributor to the total aggregate risk; summarizing the principal leading indicators that have been found through many years of experience to correlate with the potential magnitude of the U/U risk; presenting correlations of historical data that provide insight into the potential contribution from U/U risk to the total aggregate cost of a program or project; and providing a helpful but nonprescriptive table of adjustments to the calculated cost of a program or project to account for U/U risk as a function of composite leading indicators.