This document discusses various concepts of population density and their modifications. It begins by defining population density as the number of people per unit area. Several modified concepts are then presented, including physiological density, agricultural density, comparative density, critical density of population, economic density, urban density, room density, nutritional density, and settlement density. Each modifies the numerator, denominator, or both of the basic population density ratio. The document also discusses advantages and objections to each modified concept.