Sustainable sanitation and water pollution control calls for adoption of affordable and efficient wastewater treatment technologies. In the developing countries, the safe management of wastewater is not widespread. There is therefore a need for an appropriate technology that can reliably achieve acceptable effluent quality for discharge to the environment at minimal cost. Constructed wetland (CW) systems have been used as a cost effective alternative to conventional methods of wastewater treatment. However, the mechanistic understanding of the CW has not matured, while performance data that can guide design and operation of CW under tropical climate are scarce. This study explores the treatment of domestic wastewater with subsurface constructed wetlands, in order to provide performance data that can influence design and operation of CW under tropical conditions and to evaluate the processes involved with the transformation and degradation of organic matter and nutrients. The thesis contributes to performance data and getting a better mechanistic understanding about the factors influencing the performance of horizontal subsurface flow constructed wetland (HSSF-CW) treating real domestic wastewater under tropical conditions. The findings obtained in this research may prove useful towards the wider application of the constructed wetland wastewater treatment technology and the optimization of full-scale HSSF-CW.