**Abstract**

This chapter presents the fundamentals of the holography technique to form the beam in electromagnetic (EM) structures. The application of holography in leakywave 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 holographicbased 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.

**Keywords:** holography technique, leaky-wave antenna, metasurface, periodic structures, MIMO systems, phased-array antenna
