Essential oils, called volatile oils or ethereal oils, are natural metabolic secretions widespread in the most varied organs of the plant. Some of these natural products are extremely potent and precise as action due to their complex molecular substances, in which mono- and sesquiterpenes constituents predominate, and contain aromatic compounds. The technological process of obtaining volatile oil intervenes decisively in its composition and its quality. In this chapter, the general overview of essential oils, their chemistry, extraction methods, and analyses are described. In detail, the chemical composition of essential oils is influenced by biotic, abiotic, and genetic factors, which are discussed in this chapter. In addition, extraction of EOs is one of the most effort-requiring and time-consuming processes. In this chapter, different methods used for the extraction are discussed. Furthermore, chemical structures in essential oil have been provided in detail. This chapter also discusses some of the developments in chromatography for essential oils analysis starting from gas chromatography to coupled techniques.