The present chapter introduces several basic aspects of Nondestructive Evaluation (NDE). Its purpose is to explain what characterizes NDE. A brief overview of various methods used for NDE with respect to the challenges of NDE 4.0 will be given. The physical basics will be introduced for the most important methods, including X-ray imaging, thermography, ultrasonic and electromagnetic testing, for instance. Typical NDE tasks and how NDE is applied in the industrial production, service, and research and development will be discussed briefly at the end of the chapter.