Chemometrics plays a key role in PAT strategies. It is essential in understanding and diagnosing real-time processes, and keeping them under multivariate statistical control. This chapter will cover design of experiments, exploratory analysis, quantitative predictive modelling, classification, multivariate process monitoring and multi-block and multi-way analyses. The objective of the chapter is to describe chemometrics methods with a main focus on understanding, interpretation and evaluating the usefulness of the results.