USim simulates the evolutionary development of two-dimensional artificial lifeforms, which move in a liquid environment. At the beginning of the simulation there are random-generated individuals and a number of "food-squares", which are also distributed at random. The number of food-squares results from the number of existing individuals, this means that the bio-mass of the whole system is constant. Each individual has a (tree-)structure, which is defined by a simple genetic code. The ability of periodic movements results in a movement inside the liquid environment. When an individual encounters a food-square it "eats" the square, which leads to a positive change of the indivduals life-energy. When an individual reaches a certain age, the statistical probability of reproduction increases with its life-energy. The (non-sexual) reproduction is managed by laying "eggs", which are symbolized by yellow squares. Each egg reduces the life-energy of an individual and has to be recharged by consuming additional food-squares. Every new individual inherits the genetic code from its parent, eventually including some minor mutations. An individual dies when it reaches a certain age or when its life-energy has decreased to zero.