Abstract: The invention provides an assay or reaction apparatus of modular construction permitting true random operation at the discretion of the user. The apparatus is an automated multi-test capability assay or reaction apparatus (1) in modular form, comprising a reagent and an assay sample storage module (2,3), an incubation module (11), a reaction detection/measurement module (17), and a module (16) for independently and, as desired, both transporting an incubation container (58) between the incubation module (11) and reaction detection/measurement module (17), and transferring reagent(s) and/or assay sample(s) between different modules without reagent(s) and/or assay sample(s) intended for different tests or reactions.