The present paper focuses on the methods of derivative contract pricing. The basic differential equation of the popular Black-Scholes model for option contract pricing is derived. Furthermore, its less known modifications by Merton and Garman and Kohlhagen are pointed out. The paper refers to the significant drawbacks and limitations of the option pricing models that are based on constricting and unrealistic assumptions that often fail in comparison to the real market data. Attention is paid to the most serious problem, namely the issue of constant volatility, which is considerably disrupted in practice. Models implementing both stochastic and deterministic volatility in the original model are pointed out, their output being a more accurate option contract price.