What is humidity? Why is CO2 measured? How is the accuracy of differential pressure stated? Why measure water activity? What does the use of instruments in potentially explosive atmospheres mean? ROTRONIC has been working in the field of humidity for more than 50 years now, resulting not only in a steady growth in know-how, but also in know-what and know-why. It is time to pass on this knowledge little by little. The purpose of this chapter is to shed light on the subject and give interested readers an opportunity to clarify possible questions.