This video presents the formal definition of the power-spectral density (PSD) of a wide-sense stationary (WSS) process, and its inverse relationship, both through discrete-Time Fourier Transform (DTFT) pairs. The video presents the key properties of the PSD, many of which are related to properties of the Fourier transform, but also some key conceptual properties such as positivity, total power, and being a real function. Several examples worked examples for calculating PSDs are presented, including a detailed analysis of dealing with autocorrelation sequences (ACSs) that have a periodic component, as well as a non-periodic component.