Specific metabolic roles of P450-dependent monooxygenase systems are determined by enzymatic properties and substrate specificity of P450s, the terminal enzymes of the electron transfer chain. On the other hand, molecular diversity has also been reported for NADPH-cytochrome P450 reductase, cytochrome b₅, and cytochrome b₅ reductase in plants. Several lines of evidence indicate that the electron transfer components for plant P450 reactions have specific physiological roles. In this review, we describe the current status of knowledge of the biochemistry, molecular biology, gene regulation, and molecular diversity of plant P450-related electron transfer components and summarize possible individual physiological roles of the diversified P450 electron transfer systems in plants.