Technical Abstract: The aim of this study was to quantify the role of water availability on crop and weed interactions and weed-induced crop losses for processing tomato grown in Southern Italy. Field experiments were carried out during 2008 and 2009, for a two year period. Two levels of water availability (rainfed and irrigated) were imposed to compare weed competitive effects under irrigated and rainfed conditions on tomato. Although the decline in tomato yields by weed interference was a direct function of water applied (rain + irrigation), the relative effect of weed biomass on competition appeared to increase under drought, relative to irrigated conditions. Because of the importance of irrigation in tomato processing, and the increasing cost of water, quantification of weed-induced production losses as a function of water availability may provide useful information for land managers regarding the cost and benefits of weed control if water becomes scarce.