Classical theories of population growth and inter-species relations are briefly reviewed. It is shown how variations in feeding habits, reproductive patterns, and other aspects of population behaviour have required modifications of the basic theory in order to make it applicable to particular species. The role of the environment in population theory is also discussed, and this opens the way to a consideration of theoretical aspects of the ecology of natural populations. The latter are admittedly too complex to encourage the application of theory in its purest and most complete form. Practical attempts along these lines have simplified the problem by dealing with ecological groups—phytoplankton, zooplankton, etc.—rather than individual species. A review is presented of the methods, aims, and results of theoretical ecological investigations of the last few years. The significance of such work with respect to higher members of the food chain is discussed, and the paper concludes with remarks on some of the unsolved problems of theoretical analysis.