This review is devoted to an investigation of the pattern formation problem in mobile micro-organism populations. This pattern formation is due to the nonlinear character of the processes which control the behavior of an individual organism. Various examples of the pattern formation (population waves, swarms, Rayleigh--Taylor cells, ``green holes'', etc.) are reviewed in detail. It is demonstrated that the stability of these patterns is due to the interaction between organism and environment. The importance of investigating the transformation from random motion of individual cells to determinate behavior of cellular collectives for progress in bioengineering is discussed.