This chapter covers the basics of generalised linear mixed models in the univariate context of single-species distribution modelling. The chapter starts by discussing how species distribution models relate to the theory on environmental species niches. The modelling part of the chapter first introduces the linear model, then moves to generalised models, then to mixed models with both fixed and random effects, and finally describes how the explained variance can be partitioned among the explanatory variables. The applied part of the chapter uses both simulated and real data to illustrate how the R-package HMSC-R can be used to analyse generalised linear mixed models. While these analyses are rather standard and could also be conducted with many other packages, the reader is encouraged to go through them, as they provide the simplest way of becoming familiar with the syntax of HMSC-R.