This introduction to the 157th Faraday Discussion describes features of bimolecular reaction and photodissociation in gases and liquids and at interfaces. Two unifying ideas are the concepts of a transition state on a single potential energy surface and of a conical intersection between two surfaces, the former being important in bimolecular reactions and the latter often being important in photodissociation. State-resolved studies of the reactions of methane and its isotopologues with F, Cl, and Br illustrate many aspects of bimolecular reactions including the ability of excitation in vibrational modes to enhance or inhibit a reaction and to control the cleavage of selected bonds. There are clear parallels between those gas-phase reactions and the dissociative chemisorption of methane and its isotopologues on metal surfaces. Similarly, features such as relative reaction rates and energy disposal patterns observed in gas-phase reactions largely carry over into reactions in solution. New experiments comparing photolysis in the gas phase and in solution show that there are again many similarities in the processes in the two environments. Although the influence of the surroundings in those cases is subtle, there are situations in which the surroundings can produce a much larger effect on the photolysis by hindering the dissociation and initiating isomerization.