Photopolymerization Reactions and Photoinitiators: Backgrounds. Backgrounds in Photopolymerization Reactions: A Short Overview -- Photoinitiating System -- Photoinitiators: Structures, Excited States, Reactivity, and Efficiency. Cleavable Radical Photoinitiators -- Two-Component Radical Photoinitiators -- Cationic Photoinitiating Systems -- Anionic, Photoacid, and Photobase Initiating Systems -- Reactivity of Radicals Toward Various Substrates: Understanding and Discussion -- Role of Experimental Conditions on the Performance of a Radical Photoinitiator -- Reactivity and Efficiency of Radical Photoinitiators -- High Performance Photoinitiating Systems: Achievements, Trends, Challenges, Opportunities and Applications. Design of Photoinitiators for Enhanced Performance: A Mechanistic Approach -- Multicomponent Radical Photoinitiating Systems for Enhanced Reactivity -- Photoinitiating Systems for Free Radical Promoted Cationic Polymerization -- Photoinitiators for Novel Specific Properties -- Industrial Photoinitiators: A Brief Overview -- Photoinitiators for Specific Reactions and Traditional or Emerging Innovative Applications. Photoinitiators and Light Sources: Novel Developments -- Photoinitiators for Controlled/Living Polymerization Reactions -- Photoinitiators in Specific Polymerization Processes -- Photoinitiators for the Curing of Thick or Filled Samples -- Photoinitiators in Various Sectors of Industrial Applications.