Cyber-physical systems (CPSs) are used in many mission critical applications, such as automobiles, aircraft, and medical devices; therefore, it is vital to ensure that these systems behave correctly. Testing is required to ensure correct behavior for CPSs, but traditional methods to evaluate test data can be time consuming and unreliable. New technologies, such as property monitoring, can automatically evaluate measurements of system behaviors against requirements; however, most property monitoring methods rely on the availability of formal system requirements, often in the form of temporal logic formulae, which can be challenging to develop for complex applications. This talk describes industrial CPS challenges in defining formal requirements, presents examples from testing results for an automotive fuel cell application, and suggests possible directions for solutions.