In this paper, we present a unified framework for decision making under uncertainty. Our framework is based on the composite of two risk measures, where the inner risk measure accounts for the risk of decision given the exact distribution of uncertain model parameters, and the outer risk measure quantifies the risk that occurs when estimating the parameters of distribution. We show that the model is tractable under mild conditions. The framework is a generalization of several existing models, including stochastic programming, robust optimization, distributionally robust optimization, etc. Using this framework, we study a few new models which imply probabilistic guarantees for solutions and yield less conservative results comparing to traditional models. Numerical experiments are performed on portfolio selection problems to demonstrate the strength of our models. Qian, Wang and Wen: A Composite Risk Measure Framework for Decision Making under Uncertainty 2The existence of the uncertain parameters distinguishes the problem from ordinary optimization problems and has led to several decision making paradigms.One of the earliest attempts to deal with such decision making problems under uncertainty was proposed by Dantzig (1955), where it was assumed that the distribution of ξ is known exactly and the decision is chosen to minimize the expectation of H(x, ξ). Such an approach is called stochastic programming. Another approach named robust optimization initiated by Soyster (1973) supposes that all possible values of ξ lie within an uncertainty set, and the decision should be made to minimize the worst-case value of H(x, ξ). Stochastic programming and robust optimization models can be viewed as two extremes in the spectrum of available information in decision making under uncertainty. There are models in between these two extremes. For example, distributionally robust models (Scarf et al. 1958, Dupačová 1987, Delage and Ye 2010 take into account both the stochastic and the robust aspects, where the distribution of ξ is assumed to belong to a certain distribution set and the worst-case expectation of H(x, ξ) is minimized. There are also various models which minimize certain risk (Rockafellar and Uryasev 2000, Gaivoronski and Pflug 2005, etc.) or the worst-case risk (El Ghaoui et al. 2003, Zhu and Fukushima 2009, etc.) of H(x, ξ). We will present a more detailed review of these models in Section 2. In addition to the study of individual models, there have been recent efforts to seek connections between different models and to put forward more general models. For instance, Bertsimas and Brown (2009) and Natarajan et al. (2009) show that uncertainty sets can be constructed according to decision maker's risk preference, Bertsimas et al. (2014) propose a general framework for data-driven robust optimization, and Wiesemann et al. (2014) propose a framework for distributionally robust models. While some models have demonstrated their effectiveness in practice, there are still some ignored issues in the existing literature: ...