In this important new Handbook, the editors have gathered together a range of leading contributors who introduce the theory and practice of multilevel modeling. This Handbook establishes the connections in multilevel modeling, bringing together leading experts from around the world to provide a roadmap for applied researchers linking theory and practice, as well as a unique arsenal of state-of-the-art tools. It forges vital connections that cross traditional disciplinary divides and introduces best practice in the field. - Part I establishes the framework for estimation and inference, including chapters dedicated to notation, model selection, fixed and random effects, and causal inference. - Part II develops variations and extensions, such as nonlinear, semiparametric and latent class models. - Part III discusses missing data and robust methods, assessment of fit and software. - Part IV consists of exemplary modeling and data analyses written by methodologists working in specific disciplines. Combining practical pieces with overviews of the field, and usefully supported by a companion website, this handbook is essential reading for any student or researcher looking to apply multilevel techniques in their own research.