This chapter introduces the statistical concept of indexing as a suitable tool to address some of the complexities of environmental mixtures. With indexing, individual components of the mixture are summarized into one or more summary scores, which can simplify the description of the mixture and be included in regression modeling, overcoming some of its major limitations in this context. Particular emphasis is given to the framework of Weighted Quantile Sum (WQS) regression, a supervised indexing approach specifically designed for the assessment of the health effects of environmental mixtures. In addition to presenting the properties, implementation, and interpretation of WQS regression, the chapter also includes a discussion of several extensions of its framework such as quantile G-computation.