Weeds reduce agricultural productivity by competing for resources, and weed management is one of the largest costs faced by crop growers. Weeds are constantly evolving, and changes in weed types and their characteristics require ongoing adaptation of management. Farming systems also evolve, introducing new weed management challenges and opportunities. This dynamic nature of weed management often leads to shifting demands for research, development and extension specific to weeds and local farming systems. This study, funded by the Grains Research and Development Corporation (GRDC) and Cotton Research and Development Corporation (CRDC), aims to inform R&D investment priorities and industry at a broader scale. Given the wide range of weeds, agroecological zones (AEZs), impacts and management demands across variable seasons, it is not simple to identify where and in what form the largest costs are incurred. The last major study of the distribution and economic impact of weeds in Australian cropping systems (Llewellyn et al., 2016) began more than a decade ago and drew upon data from the period 2010 to 2014. The study focused only on grain production, and the overall cost of weeds to Australian grain growers was estimated to be $3300 million. The results of this new study represent the most comprehensive national analysis of the cost of weeds to Australian broadacre crop production. The study covers the 14 major GRDC-defined agroecological zones across the western, southern and northern regions, and the major crop types of wheat, barley, oats, canola, pulses, grain sorghum and – new to this study – cotton. This report outlines the study methods and then presents the results for the cost of weeds in grain crops, cotton crops and, finally, the cost for grain and cotton crops combined. The analysis is based on a modified, broadened and updated version of the national weed impact model used in the 2016 study. It includes the costs of yield loss due to in-crop weeds, water and nutrient use by fallow weeds, weed contamination, off-target herbicide impacts and weed control. Weed control costs, such as herbicide and non-herbicide practices, include seed technology costs attributable to weed control. Inputs used to represent cropping and farms in each AEZ have been informed by newly available data sources, including the GRDC Farm Practice Survey results; national herbicide resistance paddock survey data, including weed presence and density assessments; proprietary Kynetec annual herbicide farmer panel data; Australian Bureau of Statistics (ABS) production data; GRDC National Variety Trial yield results; Australian Pesticides and Veterinary Medicines Authority (APVMA) herbicide sales values; regional crop planning guides; and a panel of regional agronomy and weed management advisers, including cotton weed management experts.