This presentation will highlight the scientific and computational challenges facing the Fusion Energy Sciences research area. Major progress in magnetic fusion research has led to ITER – a multi-billion dollar burning plasma experiment supported by seven governments (EU, Japan, US, China, Korea, Russia, and India) representing over half of the world’s population. Currently under construction in Cadarache, France, it is designed to produce 500 million Watts of heat from fusion reactions for over 400 seconds with gain exceeding 10 – thereby demonstrating the scientific and technical feasibility of magnetic fusion energy. It is a truly dramatic step forward in that the fusion fuel will be sustained at high temperature by the fusion reactions themselves. While many of the technologies used in ITER will be the same as those required in an actual demonstration power plant (DEMO), further science and technology is needed to achieve the 2500 MW of continuous power with a gain of 25 in a device of similar size and field. Advanced computations in tandem with experiment and theory are needed to harvest the scientific knowledge from ITER and leverage its results. The associated research demands computational tools and techniques that aid the acquisition of the scientific understanding needed to develop predictive models which can prove superior to extrapolations of experimental results. Reliable modeling capabilities in Fusion Energy Sciences are expected to require computing resources at the petascale (10¹⁵ floating point operations per second) range and beyond to address ITER burning plasma issues. This provides the key motivation for the Fusion Simulation Program (FSP) – a new U.S. Department of Energy initiative supported by its Offices of Fusion Energy Science and Advanced Scientific Computing Research -- that is currently in the program definition/planning phase. The primary objective of the FSP is to enable scientific discovery of important new plasma phenomena. This requires developing a predictive integrated simulation capability for magnetically-confined fusion plasmas that are properly validated against experiments in regimes relevant for producing practical fusion energy. Substantive progress on answering the outstanding scientific questions in the field will drive the FSP toward its ultimate goal of developing the ability to predict the behaviour of plasma discharges in toroidal magnetic fusion devices with high physics fidelity on all relevant time and space scales. From a computational perspective, this will demand computing resources in the petascale range and beyond together with the associated multi-core algorithmic formulation needed to address burning plasma issues relevant to ITER. Even more powerful supercomputers at the “exascale” (10¹⁸ floating point operations per second) range and beyond will be needed to meet the future challenges of designing a demonstration fusion reactor (DEMO). Analogous to other major applied physics modelling projects, plasma physicists will need to closely collaborate with computer scientists and applied mathematicians to develop advanced software that is validated against experimental data from tokamaks around the world. Specific examples of expected advances which are needed to enable such a comprehensive integrated modelling capability and possible “co-design” approaches will be discussed.