Weeds have a wide variety of impacts on society, the environment and the economy. Some of the economic impacts are benefits but most are costs. Combellack (1987) valued the economic cost of weeds in 1981-82 to be $2096m. New methods of weed control and new techniques of farm management have since been developed, and new weeds have invaded. Therefore, the current cost of weed impacts cannot be readily compared to those of 1981-82. This paper attempts to estimate the economic costs of weeds in agriculture across Australia. But further, it offers an economic framework to help consider the problems that weeds create, and the generation and use of information to resolve those problems. A stochastic simulation model was developed to estimate the economic impact of weeds and to particularly account for variability in the cost estimates. The total annual economic loss to Australian agriculture ranged from $3400m to $4400m, with a mean loss of $3900m.