Solar-powered membrane distillation (SP-MD) desalination systems have been proposed as promising technology in off-grid areas with strong solar radiation. However, despite the various SP-MD pilot systems constructed and tested over the past two decades, such systems have not been commercialized or implemented on a large scale yet. The objective of this study is to provide a comprehensive review and assessment of established SP-MD systems and to identify common and contrasting features among them. The study focuses mainly on SP-MD systems that had been tested under field conditions for at least several days and which are either fully or partially powered by solar energy. The systems were first reviewed in terms of their physical properties and operating conditions. The systems were then evaluated in terms of several performance indicators, such as membrane flux achieved, energy consumption, gained output ratio (GOR), performance ratio, and the energy recovery scheme applied. A numerical model was constructed and run to evaluate how a key performance indicator, GOR, can be enhanced by certain operating parameters. The results of this study are used to suggest new areas of improvement for future SP-MD projects. (C) 2011 Elsevier B.V. All rights reserved.