I will discuss how to quantify perturbative theory uncertainties with theory nuisance parameters (TNPs). I first discuss the fundamental needs and motivations for a rigorous treatment of theory uncertainties and their correlations for precision analyses. I briefly recap why traditional scale-variation based approaches just don't cut it. I will then give a pedagogical introduction to the basic ideas and concepts behind the TNP approach and how it provides meaningful theory uncertainties that reflect our knowledge, correctly account for correlations, and allow for a meaningful statistical interpretation. To illustrate its application I will use qT resummation as a concrete example.