Weeds significantly reduce agricultural productivity and pose a serious threat to food production and economic livelihoods. Weeds aggressively compete with crops for essential resources such as water, nutrients, and sunlight, thereby adversely affecting crop performance. Weeds represent a critical and persistent challenge in Iranian agriculture, causing an estimated 25% to 30% reduction in overall crop yields. Although traditional methods such as plowing and weeding, combined with modern technologies including drones, biological control, and the development of herbicide-resistant crop varieties, are employed in Iran, substantial yield losses persist. In Iran, weeds are recognized not only as a major threat to crop productivity and food security but also as disruptors of natural ecosystems, contributing to biodiversity decline. The increasing demand for agricultural products, driven by population growth, has led to the expansion of cultivated areas, which in turn has created additional habitats conducive to weed proliferation. Consequently, both the diversity and density of weeds have increased, complicating their management, particularly for vital crops such as wheat. This complexity is further exacerbated by changes in weed species composition, the evolution of herbicide resistance, and variability in climatic conditions. A comprehensive understanding of weed biology enables identification of critical growth stages and reproductive periods, facilitating targeted control measures to prevent weed establishment and seed production. This chapter explores the biology of key weed species affecting major crops in Iran, including wheat, rice, corn, chickpea, and rapeseed, and presents successful research findings that can guide the future of weed management in the country.