The Extreme Universe Space Observatory (EUSO) is designed to detect the highest energy particles in the Universe by observing the fluorescence and (reflected) Cherenkov light produced when these ultrahigh energy cosmic rays (UHECR)
traverse the Earth's atmosphere. Unlike existing cosmic ray observatories, EUSO will view the atmosphere from above. A number of pathfinder missions have been completed or are in development in order to evaluate the potential of this
method, including a prototype instrument flown aboard a stratospheric balloon, a similar instrument co-located with the Telescope Array, an improved instrument to be flown aboard long-duration super pressure balloon, as well as a precursor instrument called Mini-EUSO, which will fly aboard the international space station. Here we describe a relatively new general-purpose software framework,
known as Offline, which facilitates detailed simulation and reconstruction of UHECR events and laser and flasher test shots observed by the various EUSO pathfinders. This new software is based on open-source codes developed over
roughly a decade by a collaborative effort of several particle astrophysics and high energy physics experiments, and is designed with the needs of large
collaborations in mind. We briefly explain the machinery used to manage user contributions to simulation and reconstruction algorithms, organize an abundance
of configuration data, and provide access to time-dependent information on detector and atmospheric properties. We show a number of examples of simulation and reconstruction of the EUSO pathfinder data using this software.