Probing Molecular Clouds Their Density and Structure

Knowledge of physical conditions in molecular clouds is a prerequisite for deriving the distribution of molecular material in the Milky Way and other galaxies, understanding star formation within these dense regions, the stellar initial mass function, and appreciating the eeects of interaction of young stars with their natal environment. In this lecture, I review some of the basic techniques for determining physical parameters of interstellar molecular clouds, emphasizing measurements relating to mass, column density, and density. These studies provide considerable information concerning the relationship of molecular clouds and star formation, and are beginning to suggest to us what is the actual nature of these clouds.