Belle II is a next generation B factory experiment that will collect 50 times more data than its predecessor, Belle. The higher luminosity at the SuperKEKB accelerator leads to higher background levels and requires a major upgrade of the detector. As a consequence, the simulation, reconstruction, and analysis software must also be upgraded substantially. Most of the software has been redesigned from scratch, taking into account the experience from Belle and other experiments and utilizing new technologies. The large amount of experimental and simulated data requires a high level of reliability and reproducibility, even in parallel environments. Several technologies, tools, and organizational measures are employed to evaluate and monitor the performance of the software during development.