Among approximately 250,000 species of plants worldwide, about 8,000 species behave as weeds, and 250 of them are found important for world agriculture. Primarily, arable weeds reduce crop yield by competing for resources such as light, water, nutrients, or space. They also contaminate crops reducing yield quality, serve as hosts for crop diseases or pests, and interfere with harvest. Some weed species produce chemical substances which are harmful to crop-plants, animals, and humans. Despite the negative impact, weeds have significant functions in the agroecosystem: they constitute food for different trophic groups, participate in carbon sequestration and nutrient cycling, and ensure soil stability. Finally, weeds are a part of biodiversity and they are valued for their cultural traces and aesthetic qualities.

The negative and positive aspects and functions of weeds in agroecosystems are closely related to the chemical processes occurring in their organisms and their chemical composition. Specific chemical compounds determine their beauty, attractiveness, or harmfulness to animals. Weeds can be reservoirs of nutritionally valuable elements and compounds, as well as accumulators of heavy metals and other toxic substances. Due to their specific chemical properties, some weed species can be used by various industries.

This Special Issue aims to bring together original research articles covering any aspect of weed chemistry. Original articles, as well as review articles describing the state of the art and the new developments in the field, are welcomed.

Potential topics include but are not limited to the following:

• Chemical and biochemical diversity of weeds - differences between weed species in chemical composition
• The influence of environmental and agrotechnical factors on the chemical composition of weed plants
• The contribution of weeds to the element cycles
• Chemistry of weed resistance to herbicides
• Allelochemicals released from weed plants or their residues
• Chemical compounds that determine the interactions between weeds and animals/insects
• Weed-derived chemical compounds used as biostimulants and biopesticides
• Minerals and organic compounds contained in weeds which are valuable for chemical, pharmaceutical, cosmetic, food, and beverage industries
• Weed-derived substances which enhance feed quality or are toxic to farm animals and humans
• Methods and techniques of isolation of target chemical compounds from weed biomass
• The use of weed chemistry in bioindication of the environment status and contamination
• Potential of weeds for phytoremediation of habitats contaminated with heavy metals and other toxic substances