The physical devices and interactions that we design and prototype also need to be coded and engineered – made into resilient, functional and usable products.  In this chapter we look at some of the special issues that arise when developing the computational side of physical-digital products.  The chapter starts by looking at models which address some of these issues, in particular the continuity in time and space of physical phenomena and the way in which in many sensor-rich systems the impact on, and hopefully benefit to, people may be separated from the actions that were originally sensed.  It then moves on to consider the development of physical systems, which are often distributed on small devices across space and involve raw electronics.  Crucially developers need to deal with issues that are absent or less common in more purely digital systems, including dealing with detailed timing and delays and often grubby pragmatic considerations.

Keywords: computational modelling, physical-digital products, space-time, physical phenomena , sensor-rich systems, physical systems