In discussions of ASCI, the high-profile procurements of large computers frequently figure prominently. However, from the outset of the ASCI program, applications have been recognized as the driver. These applications feature complex, multi-physics simulations of natural phenomena that generate massive data sets as output. As we have moved from computing systems dominated by parallel vector processing to massively parallel processing we have designed new applications from the ground up to take advantage of the new capabilities. Early payoffs from this effort include running problems that are one to two orders of magnitude larger than any we have been able to run in the past. With these larger problems, we are begining the computational exploration of domains in physics, chemistry and engineering that were previously closed. As we write these codes, issues associated with languages, debuggers and visualization tools have quickly risen to the surface. The process of running large problems has strained the computational infrastructure almost to the breaking point but indicates the direction for future work.