An apparatus for the cryogenic distillation of air includes an assembled unit that has a first distillation column module within which is provided a cryogenic distillation column; a heat exchange module within which is provided heat exchange means for cooling column feed air to a cryogenic distillation temperature; and at least one further processing unit. The or each distillation column, the heat exchange means, and the or each further processing unit are operationally interconnected, and the assembled unit is suitable for transportation to and erection at the site for a cryogenic air separation plant. Construction of a cryogenic air distillation plant is simplified resulting in a reduction in construction cost and time of construction. In addition, as components of the apparatus are self-contained within modules, the risk of contamination of the internal components is reduced, thus improving control of the quality of the construction.