System trustworthiness is therefore a combination of several key software attributes: reliability, safety, security, availability, performance, fault-tolerance, and privacy. Some of these attributes can be directly measured, some cannot. For example performance and availability can be numerically measured; safety and security cannot. Further, several of these attributes may conflict, such as security and performance. Therefore to demonstrate that the software of a system can be trusted, it requires a combination of qualitative arguments concerning the level achieved for some attributes in combination with the numerical (quantitative) scores measured for others. In order to understand the trustworthiness and security of a software system, we first need to understand its reliability and fault tolerance. Therefore I will propose here the need for new thinking in these two areas.