The last eight chapters (18–25) have focused on the somewhat narrow field of pathogenic microbiology, necessarily overlooking many other interesting and useful aspects of microbiology and microorganisms. This last chapter emphasizes microbial activities that help maintain, sustain, and control the life support systems on the earth. This subject is explored from the standpoints of (1) the natural roles of microorganisms in the environment and their contributions to the ecological balance, including soil, water, and mineral cycles, and (2) the artificial applications of microbes in the food, medical, biochemical, drug, and agricultural industries.