Perforated fins are provided to improve the capabilities of fin and tube type heat exchangers, and to adapt them for flow outside of the tube that is essentially parallel to the axis of the tube. The fins are made of a thermally conductive material, such as metal, with perforations in the fins. Fins can be of any shape. Typically, one or more tubes or binding posts pass through the fins. The fins are attached to the tube or post by press fitting, furnace or torch brazing, welding, or other method of mechanical bonding. The perforations allow heat exchange with the contents of a tube of a fluid flowing essentially parallel to the axis of the tube, in contrast to conventional fin-tube heat exchangers. The fins may also be bonded to a post or other securing means and inserted into the inside of a tube or other hollow body to improve efficiency of heat exchange. In addition, the fins may carry a catalyst, optionally carried on a washcoat or similar treatment to increase surface area.