When confined light interacts strongly with molecular electronic transitions, the states of the system become hybrid between light and molecules: polaritons. Resorting to molecular polaritons is a valuable strategy to manipulate the energy levels of molecules and the reaction pathways in photochemical reactions. In this review, we first adapt the conceptual tools of traditional photochemistry to the case of polaritons, highlighting the main analogies and differences with respect to purely electronic transitions. We then make use of such tools to understand the basic idea of a series of selected and promising applications aimed at manipulating photochemical processes with quantum light.