A central problem for regulators and risk managers concerns the risk assessment of an aggregate portfolio defined as the sum of d individual dependent risks (Xi). This problem is mainly a numerical issue once the joint distribution of (X1,X2,...,Xd) is fully specified. Unfortunately, while the marginal distributions of the risks (Xi) are often known, their interaction (dependence) is usually either unknown or only partially known, implying that any risk assessment of the portfolio is subject to model uncertainty.