This chapter covers the most challenging aspect of quasar absorption line studies – estimating the densities, dynamic conditions, metallicities, ionization conditions, and general cloud properties (masses, sizes, stability) that match the observed data. The techniques have evolved from single-cloud single phase models that were simply constrained by the measure column densities, to kinematically complex, multi-cloud multiphase models that are constrained by absorption profile morphologies on a pixel-by-pixel basis. In this chapter, we cover the modeling methods by describing them in order of complexity and ambition. These methods are the chi-square method, the density-metallicity locus method, and Bayesian approaches, including Markov Chain Monte Carlo (MCMC) methods and profile-based multiphase Bayesian modeling. Methods are discussed and examples are provided, but modeling absorbers is a scientific artform that requires a deep intuition that can only be developed through lots of practice.