System Validation is the field that studies the fundamentals of system communication and information processing. It allows automated analysis based on behavioural models of a system to see if a system works correctly. We want to guarantee that the systems does exactly what it is supposed to do. The techniques put forward in system validation allow to prove the absence of errors. It allows to design embedded system behaviour that is structurally sound and as a side effect enforces you to make the behaviour simple and insightful. This means that the systems are not only behaving correctly, but are also much easier to maintain and adapt. ’Requirements by modal formulas' is the third course that shows you how to specify requirements for the automata in order to establish the correct relation between the requirements and the behaviour of the system. Reading material. J.F. Groote and M.R. Mousavi. Modeling and analysis of communicating systems. The MIT Press, 2014.