The purpose of this book is to help extension workers and farmers to identify plants that are weeds or are considered to have the potential to become weeds in the upland cropping systems of Cambodia. Not all of the plants included are currently regarded as weeds, and many of them have useful purposes. However, with changing agricultural practices such as reduced or zero tillage, plant species currently controlled by ploughing could become weeds. These include perennial species and those with underground stems and rhizomes. Some important species known to be weeds of upland crops have not been included in this volume because of our inability to identify them or to obtain suitable photographic images. These will be included in a future revised edition.