**1. Introduction**

This Special Issue entitled 'Plant Essential Oil with Biological Activity' comprises 15 papers covering a wide range of different aspects, ranging from the biochemical characterization of some PEOs and their biopharmaceutical activities and possible agri-food and medical applications. In particular, some important published research papers have manipulated different aspects related to the biological investigation of crude essential oils (EOs), whereas other papers were related to the study of their specific single constituents. Some other research papers focused on the comparative chemical profiles of different species/ecospecies of the same plant. In addition, some studies have highlighted the seasonal changes in the production and utilization stages of some PEOs. Furthermore, some research studies are related to the use of EOs for the control of serious phytopathogens. On the other hand, our Special Issue has also covered some important review papers covering different aspects of bio-pharmaceutical properties and agri-food applications.
