For a batch reaction, the conditions within a reactor (the concentrations of materials and products) change with time. In contrast, for a flow reaction, the conditions within a reactor remain constant with time, but the conditions differ at different locations within the reactor. The reaction time for a flow reaction can be controlled by controlling the residence time in the flow reactor. Quench-flow method, in which a quenching reagent is added at the exit of the reactor to quantify the products with the residence time being varied, is effective for analyzing flow reactions. In a reaction involving an intermediate that easily decompose, a product can be obtained in a high yield only when a quencher is added in a time range during which a sufficient amount of the intermediate is produced and most of the intermediate is yet to decompose. Although a batch reactor can be used if the reaction is slow, a flow reactor is needed for a fast reaction (high-resolution reaction time control). Because the reaction rate depends on temperature, the temperature–residence time map, which can be obtained by the quench-flow method at different temperatures, is effective in optimizing the flow reaction conditions to obtain the product in a high yield.