This chapter delves into the characteristics of various liquid and gaseous fuels, focusing on their properties and their suitability for different internal combustion (IC) engines. Liquid fuels such as gasoline, diesel, kerosene, and ethanol are examined in terms of energy content, volatility, combustion characteristics, and storage requirements. Gaseous fuels, including natural gas, propane, and hydrogen, are analyzed for their calorific value, combustibility, and ease of handling. Essential properties required in an ideal fuel for IC engines include high energy density, appropriate volatility, minimal residue production, and favorable combustion efficiency, ensuring optimal engine performance, reduced emissions, and operational reliability. The chapter outlines the specific requirements of different IC engines, such as spark-ignition (SI) and compression-ignition (CI) engines, where SI engines benefit from fuels with high octane ratings to prevent knocking, and CI engines require fuels with high cetane numbers for efficient ignition. Alternative fossil fuels, such as liquefied petroleum gas (LPG) and compressed natural gas (CNG), are also discussed for their role in reducing dependency on traditional fossil fuels. The chapter further explores bio-blends, including biodiesel, bioethanol, and biogas, highlighting their properties like renewability, lower greenhouse gas emissions, and compatibility with existing engine technologies. Biodiesel’s higher lubricity and lower sulfur content make it favorable for CI engines, while bioethanol’s high octane rating suits SI engines. This chapter underscores the potential of these bio-blends to enhance energy security and environmental sustainability, demonstrating their growing importance in the future of IC engine fuel technology.