Presently, there is an increasing evidence concerning the bio-activities of various plant extracts and individual compounds and the probability of using them in different therapeutic applications, including the development of new antimicrobial agents. The essential oils could exhibit microbicidal activity, potentiate the activity of conventional antibiotics or act as virulence attenuators, by inhibiting the production of toxins by the microbial cells, or affect the ability of microorganisms to adapt to the environmental conditions provided by the host, without interfering with microbial growth, so, without exhibiting selective pressure for the occurrence of resistant strains. Besides their biological activity, the essential oils are eco-friendly and biodegradable. The purpose of this book is to present our data on obtaining, screening and selection of some essential oils with optimal antimicrobial/antibiofilm activity, by using standard, as well as new in vitro methods.