Abstract: Interactive exploration and analysis of large data sets, intelligent simulation ("cog-sim") workflows that combine interactive analysis and AI techniques with modeling and simulation, interactive preparation and debugging of large-scale scientific simulations, in-situ visualization, and application steering are all compelling scenarios for exploratory science, design optimizations, and signal processing. However, a range of technical, organizational and sociological challenges must be overcome to make these interactive workflows mainstream in HPC centers: What simulation scenarios or problem domains can benefit most from interactivity? How can we simplify the toolchain? What center policies are needed to support highly interactive workflows? The goal of this workshop is to bring together domain scientists, tool developers, and HPC center administrators to identify the scientific impact and technical challenges of highly interactive access to HPC resources.