This chapter presents the fundamentals of the holography technique to form the beam in electromagnetic (EM) structures. The application of holography in leaky-wave antennas, metasurface reflectors, and reconfigurable intelligent surfaces (RISs) is explained. Consequently, different methods to analyze and realize an EM hologram are presented. A comparison is made between forming the beam via holographic-based radiators, phased-array antennas, and MIMO systems. The thing which is common between these three is that all of them can contain a number of elements that are repeated in a fashion. However, the functionality of these elements in the three mentioned structures is totally different from each other. This concept is explained in detail in this chapter.