Hierarchical linear and nonlinear modeling (HLM) is a specialized statistical software program for analyzing multilevel and longitudinal data. It is published by Stephen Raudenbush, Anthony Bryk, and Richard Congdon and distributed by the Scientific Software International, Inc. (Chicago, IL). The program’s original versions came out in the early 1980s. The design of the program—its modeling modules and options, input specifications, and output—is in close coordination with the textbook written by Raudenbush and Bryk, Hierarchical Linear Models: Applications and Data Analysis Methods. The following entry describes the analytical options, estimation approaches, inferential methods, and operational and output features of HLM.