A solid oxide fuel cell stack (10) comprises a plurality of modules (12). Each module (12) comprises an elongate hollow member (14). Each hollow member (14) has at least one passage (32) extending longitudinally through the hollow member (14) for the flow of reactant. Each hollow member (14) has two parallel flat surfaces (16,18). At least one of the modules (12A, 12B, 12C) includes a plurality of solid oxide fuel cells (20). The solid oxide fuel cells (20) are arranged on the flat surfaces (16,18) of the modules (12A, 12B, 12C). At least one end (34) of each module (12) is connected to an end (36) of an adjacent module (12) to allow reactant to flow sequentially through the modules (12). The arrangement of the modules (12) provides compliance in the solid oxide fuel cell stack (10) and thermal and mechanical stresses in the solid oxide fuel cell stack (10) are reduced.