Assurance Based Development (ABD) is the synergistic construction of a critical computing system and an assurance case that sets out the dependability claims for the system and argues that the available evidence justifies those claims. Co-developing the system and its assurance case helps software developers to make technology choices that address the specific dependability goal of each component. This approach gives developers: (1) confidence that the technologies selected will support the system?s dependability goal and (2) flexibility to deploy expensive technology, such as formal verification, only on components whose assurance needs demand it. ABD simplifies the detection-- and thereby avoidance--of potential assurance difficulties as they arise, rather than after development is complete. In this paper, we present ABD together with a case study of its use.