The first chapter of my book is intended to very smoothly take my readers to the nine chapters ahead. In addition, this chapter is intended to provide my readers with the basic scientific materials that would explain and clarify what is coming. This chapter will always serve as the “reference chapter” that my readers will go back to whenever they require further clarifications of anything they read in the following nine chapters. This chapter includes several interesting sections including the history, background, basic structures of Depth of Anesthesia (DOA) monitors, differences between different versions of the same monitor, time delays between displayed indices and actual anesthetic state, electric devices interferences, commercially available monitors, comparing different DOA monitors, and finally alternative occipital sensor montage.