Living organisms interact with their environment, and both positive and negative interactions between organisms are driving forces in the organisation and development of ecosystems. Negative interactions like competition, which is often regarded as the most important of these forces, directly take part in selection. On the other hand, positive interactions in symbiosis, especially mutualism, and cooporation may considerably contribute to the generation of variability and complexity and to an enormous increase in fitnes. This will be elucidated discussing examples of the various types of associations. The evolution of the eucyte (eukaryotic cell) as a basic unit of life and sexual reproduction being unique for eukaryotes has been possible on the basis of symbiosis (interspecific associations) and cooperation (intraspecific associations), respectively.