This chapter provides a concise introduction to the physical concept of a field, with a focus on fluid dynamics and electromagnetic fields. It explores how the fundamental laws and properties of fields give rise to wave phenomena. The classical wave equation formulated by d’Alembert, whose solutions describe wave propagation, is derived from both Maxwell’s equations for the electromagnetic field and the Navier–Stokes equations for fluids. Additional examples are provided to demonstrate how the same wave equation emerges in the context of elastic continua, such as vibrating strings and metal bars. Key wave characteristics and phenomena relevant to measurement applications are discussed, including different types of waves, propagation behaviours, and interactions such as interference. The chapter also covers the behaviour of waves at interfaces between different media, explaining phenomena such as reflection and refraction. Snell’s law is introduced and interpreted through Huygens’ principle. Finally, the chapter explores advanced topics critical to modern measurement systems: the Doppler effect, wave polarization, the quantum description of waves via photon flux, and fundamental concepts related to LASER light sources. These elements are essential for measurement systems applications involving the generation and processing of images and waves.