This chapter explains weighting in a manner that allows us to appreciate both the power and vulnerability of the technique and, by extension, other techniques that rely on similar assumptions. Once we understand how weighting works, we will better understand when it works. This chapter opens by discussing weighting in general terms. The subsequent sections get more granular. Sections 3.2 and 3.3 cover widely used weighting techniques: cell-weighting and raking. Section 3.4 covers variable selection, a topic that may well be more important than weighting technique. Section 3.5 covers the effect of weighting on precision, a topic that frequently gets lost in polling reporting. This chapter mixes intuitive and somewhat technical descriptions of weighting. The technical details in Sections 3.2 and3.3 can be skimmed by readers focused on the big picture how weighting works.