Sometimes simple regression gives us answers that are not just wrong, but misleading. The cause is often endogeneity — a hidden link between the variable we study and the things we can’t measure. This chapter introduces instrumental variables (IV), a way to break that link and find the true causal effect. You’ll learn what an instrument is, the two rules it must follow, and how to use it with a common method called two‑stage least squares (TSLS).