Humans have utilized the bioactive principles of different plants for various beneficial physiological properties including antimicrobial properties for many centuries. However, interests of using medicinal plants declined in the 20th century with the availability of effective synthetic antimicrobial drugs. The development of microbial resistance to various drugs has accelerated research interests towards the use of phyhemicals as alternatives to synthetic drugs in the recent years. This book presents an comprehensive reviews on the antimicrobial and antiviral properties of numerous recently reported phyhemicals, and their mechanisms of antimicrobial actions. Some of the chapters have critically discussed the beneficial and adverse effects of antibacterial, and stimulatory activities of dietary phyhemicals on rumen microbial populations, and gut microbial populations of humans and animals. Microbial adaptation and resistance of microbes to phyhemicals has also been highlighted. On the applied apects, the use of phyhemicals against drug resistance microbes, to treat microbial diseases, for food preservation, to inhibit methanogenic archaea in the rumen, and to modulate lipid biohydrogenating microbial populations to increase conjugated linoleic acids in animal-derived foods have been presented in different chapters.