The importance of biofilms for the self-purification of streams and the high availability of nutrients was recognised during the last decades. The knowledge about colonisation, succession, and demographic parameters of microorganisms on biofilms, especially for heterotrophic flagellates, is still limited. One important factor determining the colonisation process of microorganisms is the flow velocity. In addition, growth rate, division rate, and mortality of the microorganisms depend on the temperature. This has may have consequences on the colonisation speed and carrying capacity on biofilms. Detailed investigations on the dynamics of biofilm-dwelling heterotrophic flagellates are still scarce even though these organisms show often the highest abundance compared to other protozoans in biofilms. Field and laboratory experiments were conducted to answer the question on how colonisation and dynamics of heterotrophic flagellates are affected by environmental parameters. These studies may serve as first steps for a better understanding of the dynamics of heterotrophic flagellates on biofilm. The book is addressed to scientists, students, and everyone how is interesting on biofilms.