The paper introduces lavaan, an R package for structural equation modeling (SEM). lavaan is designed to be a free, open-source, and commercially viable alternative to other SEM software packages. The package aims to appeal to applied researchers, educators, and statisticians by offering an intuitive and comprehensive set of tools for SEM. The paper outlines the development of lavaan, its features, and provides examples to illustrate its usage. It also discusses the 'lavaan model syntax,' which allows users to specify models in a concise and flexible manner. The paper includes two examples: confirmatory factor analysis (CFA) and structural equation modeling (SEM), demonstrating how to fit these models using lavaan. Additionally, it covers multiple-group SEM and provides methods for examining model fit and parameter estimates.The paper introduces lavaan, an R package for structural equation modeling (SEM). lavaan is designed to be a free, open-source, and commercially viable alternative to other SEM software packages. The package aims to appeal to applied researchers, educators, and statisticians by offering an intuitive and comprehensive set of tools for SEM. The paper outlines the development of lavaan, its features, and provides examples to illustrate its usage. It also discusses the 'lavaan model syntax,' which allows users to specify models in a concise and flexible manner. The paper includes two examples: confirmatory factor analysis (CFA) and structural equation modeling (SEM), demonstrating how to fit these models using lavaan. Additionally, it covers multiple-group SEM and provides methods for examining model fit and parameter estimates.