"This course will discuss the basics of photochemistry in solution and in confined systems. A comprehensive model that helps understand the behavior of molecules in solution and in supramolecular assemblies will be presented. A brief introduction to various supramolecular assemblies that are commonly used in photochemistry will be highlighted. Molecules at rest (S0) could be energized by either heat or light. The former activates molecules to upper vibrational levels (S0#) while the latter alters their electronic and/or spin configurations corresponding to S1, S2, T1, T2 etc. states. Molecules placed in upper electronic levels undergo both physical and chemical processes that are distinctly different from those in the ground electronic state. Using carbonyl, olefin and aromatic molecules as examples the various photochemical and photophysical events that organic molecules undergo will be introduced and the basic rules governing these processes outlined. The excited state behavior of molecules present in solution and gas phase could be understood on the basis of electronic and steric factors outlined in physical organic textbooks. Extension of these concepts to reactivity of molecules in well-defined restricted spaces provided by enzymes often requires additional input. The immediate environment of the reactant molecule could influence its excited state behavior (when both are nearly of the same dimension). Highly confining host systems such as micelles, cyclodextrins, cucurbiturils, calixarenes etc., similar to enzymes, have been established to alter the photobehavior of organic molecules in aqueous solution. In solid state, hosts such as zeolites, clays, and crystals exert similar influence. Independent of the host system, reactions in a confining environment could be visualized to occur within a well-defined reaction cavity that could interact with the reactant guest molecules through weak interactions."