This lecture covers the fundamentals of multiplexing, including Frequency-Division Multiplexing (FDM) and Time-Division Multiplexing (TDM). It explains how signals are combined for electronic and optical transmission, detailing Wavelength Division Multiplexing (WDM) systems, their capacity, channel spacing, and performance degradation due to crosstalk. The lecture also introduces Optical Time Domain Multiplexing (OTDM) and discusses the techniques of multiplexing and demultiplexing in optical communication systems.