Essential oils have been an integral part of human civilization for thousands of years. Their applications are highly diverse and range from fragrances to natural constituents of a healthy diet and use as active ingredients in medicinal products. Essential oils are natural complex substances (NCS) - complex mixtures of plants, animals and minerals as they occur in nature, and also as preparations derived from them. The secrets of the mutual interactions of the single substances that constitute NCS have not yet been unveiled. Synergistic, antagonistic and additive effects can all occur. Further research of such complex plant mixtures offers new options for the development of active plant-based medicines and biologically active resources for human, animal and plant life. This is especially important as mono-substance remedies, such as antibiotics, are reaching their therapeutic limits. This book provides a concise insight into essential oils as NCS, their analytic and galenic properties, their use in veterinary medicine and aromatherapy for humans, as well as their role in cosmetic and food industries.