A three-dimensional computational study on conjugate heat exchangers was conducted. Attention was specifically directed towards studying extended surfaces used to increase heat transfer. The strategy adopted in the present investigation of forced convection in a flow passage was to use the finite volume method. Our implementation incorporated a SIMPLE-based semi-implicit solution algorithm which was applied to working equations formulated within the single-phase catalog. The analysis allowed for marked changes in thermodynamic and flow properties. To justify using the proposed numerical model to simulate this conjugate heat transfer problem, we considered first a heat exchanger with a plane fin simply because experimental data are available for comparison. This validation study was followed by a study of how a newly designed fin pattern can provide increased heat transfer. The efficiency has been judged by considering several aspects, namely the span-averaged pressure drop, Nusselt number and heat flux. To better illuminate the flow and heat transfer characteristics in a flow passage bounded by two fins having wavy geometries, we have plotted solutions in a three-dimensional format.