Essential oils are secondary metabolites that are produced by plants for the purpose of protection from pests, predators, attraction of pollinators and seed dispersal. The oil is produced in different parts of the plants and contains various chemical compounds that give plants a characteristic odour and flavor. These compounds vary between different plant species and also within plants of the same species. The yield and the efficacy of essential oils also vary between and within species. The variations in the essential oil compounds, the yield and the efficacy is influenced by both exogenous and endogenous factors. Endogenous factors such as plant age, plant density, plant genotype, part of the plant where the oil is extracted brings about variability in yield, chemical composition and the efficacy of essential oils. Similarly, exogenous factors such as light intensity and temperature, water availability, altitude and latitude, soil type and composition, preparation of the plant material before the essential oil extraction, method of extraction of the essential oil and the storage conditions after extraction also have a great influence on the chemical composition, yield and efficacy of essential oils. The factors then need to be moderated in order to ensure that the essential oil produced is of the quality required. Once extraction is done, proper storage conditions to avoid oxidation is necessary as the essential oils await to be used. They should be stored in dark cool conditions in tightly closed dark glass containers to ensure long term quality. Knowledge on the growth requirements of the individual plants that produce oils is necessary so as to produce quality oil.