This talk describes the suitable protections at architecture and system level against the effects of radiation on electronic components and digital systems. After the description of the general architecture of a space avionics system, the potential solutions for each type of units constituting an on-board computer are presented through the example of real space applications: avionics bus, links, memory units, and – the main part – processing units i.e. fault-tolerant architectures. The main fault-tolerant mechanisms are overviewed, as time replication either at instruction or task level, duplex, triplex (TMR), lock-step, and a trade-off between these different solutions. Then, real case studies are analyzed.