This chapter introduces simple and multiple linear regression models – core tools in predictive modelling due to their simplicity and interpretability. These models assume the response variable is a linear function of the predictor(s), plus a noise term. The regression function gives the expected response given the predictors. The coefficient of determination, R2, measures how much of the variance in the response is explained by the model. In simple linear regression, R2 equals the square of the Pearson correlation between response and predictor; in multiple regression, it equals the square of the correlation between response and predicted values. Each coefficient in multiple regression reflects the expected change in the response for a one-unit increase in that predictor, holding others fixed. Standardising predictors lets us compare coefficient sizes. Strong collinearity between predictors increases uncertainty in the fitted coefficients. Models using only a subset of predictors may generalise better than those using all and overfitting. The squared error risk of a modelling procedure – its expected test error – can be broken down into bias, variance and irreducible noise.