A heat exchanger is formed from a metal strip (102) wound concertina fashion to provide superimposed plates (101). The plates are stamped in pairs separated by a narrow section (103) which forms a return bend for folding the two plates over one another. Each plate (101) is provided with ribs (106) shaped to guide gas flow paths through the pocket between an inlet in one corner region of the plate and an outlet in a second corner region of the plate. Corrugated zones (109) are formed in the plate (101) between the ribs to promote a whirling motion of air as it travels between the ribs flanking the corrugated zones (109).