As soon as Metropolis had completed the initial configuration of ENIAC for the new programming method, and before it was working properly, Klara von Neumann arrived to help. She had taken the leading role in converting the flow diagrams into program code, and together they worked around the clock for several weeks to get both program and machine into a usable state and to shuffle tens of thousands of cards in and out of it during Monte Carlo simulation of each exploding fission bomb. This chapter integrates the narrative of this initial “run,” of and a second batch of calculations carried out in late-1948 with analysis of the structure of the program itself. It finishes with an exploration of further Monte Carlo work run on ENIAC, including reactor simulations, simulation of uranium-hydride bombs, and in 1950 simulation of the “Super” concept for a hydrogen weapon.