Weed communities within agronomic fields are defined by the number of different species present and the relative abundance of the individual species. At any given time, three or four species within a field typically dominate the weed community, with the remaining species present at relatively low densities. A weed shift represents a change in the weed community within a field. The majority of shifts are caused by differential selection pressure placed on the weeds present in a field. Species or biotypes adapted to current weed management tactics increase, whereas weeds susceptible to those tactics decrease. The rate of change varies with the characteristics of the weed species and the diversity of the control tactics used in a field. Occasionally, weed shifts occur due to the introduction of a new species to a region. This paper will provide an overview of characteristics that allow certain plants to become invasive and expand their range by invading new regions.