The Black-Scholes pricing model is one of the most common ways to price standard American or European options. To price options, the Black-Scholes method makes a variety of assumptions about the price of the underlying stock. One assumption required by Black-Scholes is that the stock’s price will move following something called Brownian motion. Because arithmetically-priced Asian options do not follow Brownian Motion, the standard Black-Scholes pricing model does not apply.