Abstract: A heat exchanger for the exchange of heat between two media (Ma, Mb), each of which flows through a respective one of two chambers (A, B) mutually separated by a medium-impervious partition wall (5) made of thermal conductive material. The interior of each of the flow chambers, or at least of one flow chamber, is divided into a large number of medium-flow passages, which are connected in parallel with respect to the flow of medium passing therethrough. The flow passages (13, and 17) have a substantially rectangular cross-section having a flow area which is so adapted in respect of the medium flowing therethrough that the flow in the passages is substantially laminar throughout the whole length of the passages, without a central turbulent zone. The passage walls defining the flow passages comprise a highly thermal-conductive material and are formed integrally with, or in good heat-conducting contact with the partition wall (5) located between the two flow chambers (A, B).