The chapter discusses the field of photocatalysis, which utilizes photon energy to initiate chemical reactions through processes like single electron transfer, energy transfer, or atom transfer. The efficiency of photocatalytic systems depends on the ability of light-absorbing materials to execute these processes. Photoredox techniques, which use photocatalysts that function as both oxidizing and reducing agents, have gained significant attention due to their ability to work with low-energy light sources and ambient temperatures. These techniques have revitalized various synthesis realms, including radical chemistry and photochemistry, and are widely used in medicinal chemistry and natural product synthesis. Photocatalysts are also crucial in biomedicine, environmental pollution management, and water purification. Recent research has focused on improving the cost, recyclability, and environmental impact of photocatalysts. The chapter highlights the importance of understanding the fundamental principles of photocatalysis and its applications, emphasizing the potential of photochemical catalysis as a powerful tool for sustainable chemical synthesis.The chapter discusses the field of photocatalysis, which utilizes photon energy to initiate chemical reactions through processes like single electron transfer, energy transfer, or atom transfer. The efficiency of photocatalytic systems depends on the ability of light-absorbing materials to execute these processes. Photoredox techniques, which use photocatalysts that function as both oxidizing and reducing agents, have gained significant attention due to their ability to work with low-energy light sources and ambient temperatures. These techniques have revitalized various synthesis realms, including radical chemistry and photochemistry, and are widely used in medicinal chemistry and natural product synthesis. Photocatalysts are also crucial in biomedicine, environmental pollution management, and water purification. Recent research has focused on improving the cost, recyclability, and environmental impact of photocatalysts. The chapter highlights the importance of understanding the fundamental principles of photocatalysis and its applications, emphasizing the potential of photochemical catalysis as a powerful tool for sustainable chemical synthesis.