One of the most important properties of linear time-invariant (LTI) systems is the superposition principle, which states that the overall response of an LTI system to a weighted sum of signals is simply the same weighted sum of the responses to each of the individual signals. In this "Lecture Notes" column, we will provide a deeper insight into this principle. In what follows, if the superposition principle has finitely many terms, it is called the weak superposition principle, whereas, if it has infinitely many terms, it is called the strong superposition principle (SSP).