Cytochrome P450 enzymes (CYPs) are heme-containing monooxygenase enzymes that exist in nearly every living organism. They are responsible for oxidizing a wide variety of substrates in biosynthetic and detoxification pathways and are a common target for drug design. CYP-drug binding modes are traditionally characterized using optical difference spectra, but these binding assignments oftentimes miss atypical CYP-drug binding and do not account for mixtures of binding modes. Electron paramagnetic resonance (EPR) spectroscopy can shed light on CYP-drug binding. A mixture of continuous wave (CW) EPR and pulsed EPR methods provides some valuable insight into how common drugs and drug fragments interact with the active site heme. First, the two human isoforms that contribute significantly to drug metabolism, CYP3A4 and CYP2C9, are studied in complex with a variety of drugs. The results demonstrate that CW EPR parameters can reveal drug binding modes. Remarkably, this research finds an abundance of water-bridged complexes, and many of them coexist in frozen solution with complexes where the drug directly coordinates to the heme. These mixtures of binding modes have significant ramifications for drug design. Additionally, CYP3A4 is studied in the context of drug-drug interactions, looking at how common drugs, like acetaminophen, caffeine, midazolam, and carbamazepine can simultaneously occupy the active site when combined. These results find that multiple drugs occupy the active site and that they are distinct from any CYP-single drug complex. A method is established for observing human CYP drug-drug interactions with EPR, and it provides evidence of simultaneous drug binding with common drugs. In addition to human CYPs, this research examines drug binding in CYP51B1, a Mycobacterium tuberculosis CYP isoform. The results find that inhibitor-like compounds form mixtures of bound complexes, including some that retain the axial water. Overall, this research provides several new details about CYP-drug interactions. These results and observations highlight the importance of understanding and characterizing CYP-drug binding with more detailed analyses that provide information on the full range of CYP binding modes.