[Weed science at the core of 21st century issues] The goal of this paper is to capture the “weed science” dimension of various contemporary issues, agricultural and otherwise, at a time when scientific and technical resources for this discipline are dwindling. The recent adoption of transgenic crops resistant to herbicides allowed the characterization of agroecological problems related to gene flow and persistence of transgenes in the environment. However, questions concerning second generation transgenic crops are still without answers. Climate change that is affecting the adaptation zones and growth of crops is also having similar effects on weeds. Notorious weed species are already benefiting from increasing ozone and CO2 concentrations. Alternatively, weedy or invasive species are being considered as potential sources for biofuels and new crops, notwithstanding incomplete knowledge on the consequences of using these species as crops. Attempts are made to recognize the contribution of weeds to biodiversity in spite of very partial knowledge of the functional relationship between weeds and other organisms in the agroecosystem. More than ever, the knowledge of weeds is of strategic importance, these plants being at the core of sensitive ecological and economic issues.