Late emerging weeds are a concern for corn and soybean growers and their consultants because of their potential impact on crop yield and harvest efficiency and their seed production if left uncontrolled. The challenge with late emerging weeds is determining if control is justified because the weed exceeds an economic threshold and will reduce yield or that the seed production justifies control. This paper will review several factors that need to be considered when trying to assess the impact of late emerging weeds. In general, we need to consider the time of weed emergence, their survival, how successfully they grow in competition with the crop, the effect of their competition on crop yield, and their seed production.