Essential oils, a major group of phytogenic feed additives (PFA), are considered to be a cost effective and safe alternative to antibiotics as growth promotors. EOs are an important alternative to antibiotics in animal diets. The application of EOs has resulted in an improvement in the durability of raw feed materials and in the range of positive effects on domestic animals’ health and performance. PFA and EOs are expected to be a regular component of animal feed, significant for the development of the poultry and livestock industries. However, many factors influence their effects on animals. As such, standardizing and optimizing the EO composition and the quality to be applied as feed additive is necessary, which is dependent on factors affecting plant secondary metabolite production, followed by the manufacturing processing phase of feed additives, and mutual interactions with other substance from food matrix, to their absorption in small intestine. For further PFA use as reliable growth promotors, identifying the efficient bioactive compounds is required to develop methods for determination of their comprehensive modes of actions. Similarly, a clear correlation between in vitro and animal trial results is needed with special focus on standardizing the in vivo studies where efficacy of EOs is evaluated [3]. Investing in the determination of PFA mode of action could lead to their increased price in the end market and cost efficacy of their application in the animal industry. Therefore, application and further development of techniques, such as microencapsulation of EOs, is essential to standardize safety and efficacy and to provide reliable and cost effective natural feed additives.