A concept of critical period of weed competition has been introduced for more than 40 years ago. The conceptis based on the assumption that weeds are not equally harmful to a crop during the whole season and thatthere is a period in crop development in which weeds impact on the yield is the biggest. This period is calledcritical period of weed competition (CPWC), critical period of weed interference, or critical period of weedcontrol. There is a difference in CPWC between crops, but CPWC for a certain crop can vary a lot because itdepends on many factors which can affect the crop or weeds competition ability. The critical period of weedcompetition identification is essential for integrated weed control and precise planning of a weed controlstrategy as well as for rationale use of herbicides and other weed control measures.