This paper discusses the fundamental requirements of power integrity (PI), and the most important concept used in PI, the network impedance. It answers the questions commonly asked by digital designers on how a power delivery network (PDN) is represented, how PDN impedance is generally defined, what the target impedance is and its effective range, and the factors affecting impedance analysis. The paper also addresses the contributions of IC current, package inductance, and on-die capacitance to PDN impedance analysis.