When we solve a differential equation, the answer almost always depends on some numbers we chose—a friction coefficient, an interest rate, a reactor temperature. A natural next question is: “If I tweak that number a little, how much does the solution change?” This chapter is your practical guide to answering that question reliably, without numerical pitfalls like catastrophic cancellation. You will learn several ways to compute sensitivities (derivatives of a solution with respect to parameters), from simple but fragile finite differences to reliable methods like the complex step, continuous sensitivity equations, and automatic differentiation.