A heat exchanger with a plurality of stacked flat tubes and a collecting tank having a wall extending around the entire periphery of, and connected to, the end of the stacked flat tubes. A first medium may be distributed through the collecting tank and flat tubes. Internal inserts are in the flat tubes, with the inserts being bonded between the broad sides of the tubes and, in the region of connection of the tubes to the collecting tank, being configured to compensate for length changes in the stacking direction caused by temperature changes, as by recesses in connectors such as wave flanks or by corrugated wave flanks. The flat tubes with inserts such as described may be separately provided for use in manufacture of heat exchangers.