runmlwin: A Program to Run the MLwiN Multilevel Modeling Sofware From Within Stata. runmlwin is a Stata command which allows Stata users to run the powerful MLwiN multilevel modelling software from within Stata. The multilevel models fitted by runmlwin are often considerably faster than those fitted by the Stata’s xtmixed, xtmelogit and xtmepoisson commands. The range of models which can be fitted by runmlwin is also much wider than those commands. runmlwin also allows fast estimation on large data sets for many of the more complex multilevel models available through the user written gllamm command. MLwiN has the following features: Estimation of multilevel models for continuous, binary, count, ordered categorical and unordered categorical data; Fast estimation via classical and Bayesian methods; Estimation of multilevel models for cross-classified and multiple membership nonhierarchical data structures; Estimation of multilevel multivariate response models, multilevel spatial models, multilevel measurement error models and multilevel multiple imputation models