Option prices change every moment as the price of the underlying asset jumps around. To make realistic pricing models, we need a way to describe randomness that happens all the time, not just in steps. This chapter builds that language. We start with Brownian motion, then learn the rules of stochastic calculus, and finally look at the processes that drive stock prices, interest rates, and groups of related assets. By the end, you’ll have the math tools that make the Black–Scholes model and its extensions possible.