This study of heated horizontal rectangular fin array (HRFA) under natural and assisting mode of mixed convection is presented in this book. The parameters of study are fin spacing, temperature difference between the fin surface and ambient air, and flow velocity. Results are generated in the form of variation in average and base heat transfer coefficient, average and base Nusselt numbers, Rayleigh numbers, Richardson number and plausible flow patterns under natural and mixed convection. The experimental results are validated with the available data under natural and mixed convection. The flow pattern study is also carried out using a simple smoke technique and temperature mapping. Results obtained by natural convection are also compared with buoyancy assisted mixed convection. It reports that the experimental result shows significant improvement in convective heat transfer coefficient with a little energy input. The work reported in this book is an attempt to enhance heat transfer performance of HRFA heat sink using assisting mode of mixed convection. This study ends with a discussion on original contribution of recent studies and possible direction for further research.