Forward osmosis (FO) is a membrane separation process using a highly concentrated draw solution with high osmotic potential to draw water across a semi-permeable membrane from a feed source. This feed source may be seawater, wastewater or other natural or contaminated water sources. Unlike other membrane driven purification processes, the product is not clean water, but a diluted draw solution. As a result a second step is often needed to produce a pure water product. A major advantage of FO is that the low hydrodynamic pressure involved leads to lowered fouling of membranes and greater flux recovery after cleaning, as well as often providing a low energy process which can recover clean water from difficult or highly fouling sources. Selection of an appropriate and effective draw solution is essential for the practical operation of an FO process. This review will give an overview of the theoretical underpinnings of draw solution performance and a comprehensive summary of the current literature regarding the different types of draw solutions which have been investigated and their respective benefits and detriments.