The simplest definition of a weed is a plant that is a nuisance. Thus, an aquatic weed is an aquatic plant which interferes with the use of water, or in some other way constitutes a nuisance to man or hazard to human welfare. However, in contrast with single purpose systems such as an agricultural crop in which weeds are readily identified as such, water bodies frequently have more than one use, and assessment of the weediness of a plant may be confused when it interferes with one use. such as navigation, while promoting another, such as fish production (Mitchell, 1974, 1978). The value judgements made in these situations are often one-sided and based on incomplete understanding of the nature of the system, the role of the relationships involved in these situations. Sometimes, as in the case of Eichhornia crassipes in the Nile (see section 7.2.2) or Salvinia molesta on Lake Kariba (see section 7.2.5), there is general agreement that the plants in question are weeds, though it should be noted that even the most detrimental plant invasion can have some benefits and that adjustments to problems that are intransigent or too expensive to control are always possible. Keywords: Aquatic Plant, Grass, Carp, Aquatic Vegetation, Eichhornia Crassipes, Water Weed.