In occupant modelling the interest is often more on the population of occupants, rather than the individual occupants, and this calls for the so-called mixed effect or random effect models. The chapter introduces linear mixed effects models as a natural extension of the general linear model. The general approach throughout the chapter is to describe the correlation structure induced by the model and hence formulate the likelihood. The chapter discusses the REML estimator and nested random effects. The theory is illustrated using an example of clothing insulation level. The estimation of random effects is discussed using the projection theorem and again illustrated using the example.