In many cases, it is possible to derive a quantitative reliability or availability assessment for systems containing software with the appropriate use of system-level measurement-based modeling and supporting data. This paper demonstrates the system-level measurement based approach using a simplified safety protection system example. The approach is contrasted with other software reliability prediction methodologies. The treatment of multiple correlated and common mode failures, systematic failures, and degraded states are also discussed. Finally a tool called MEADEP, which is now under development, is described. The objective of the tool is to reduce the system-level measurement-based approach to a practical task that can be performed on systems with element failure rates as low as 10/sup -6/ per hour.