Consumer demand for improved quality and fresh-like food products has led to the development of new nonthermal preservation methods. High pressure processing (HPP) is currently the novel nonthermal technology best established in the food processing industry. However, many potential HPP applications would require long treatment times to ensure an adequate inactivation level of pathogens and spoilage microorganisms. High hydrostatic pressure and the addition of essential oils (EOs) have similar effects on microbial structures and thus they may act synergistically on the inactivation of microorganisms. Therefore, the combination of high hydrostatic pressure with EOs is a promising alternative to expand the HPP food industry. In this work, findings on this scarcely investigated hurdle option have been reviewed with a focus on the mechanisms involved. The main mechanisms involved are as follows: (1) membrane permeability induced by HPP and EOs facilitating the uptake of EOs by bacterial cells; (2) generation of reactive oxygen species via the Fenton reaction; (3) impairment of the proton motive force and electron flow; and (4) disruption of the protein-lipid interaction at the cell membrane altering numerous cellular functions. The effectiveness of a specific EO in enhancing the microbial inactivation level achieved by HPP treatments depends on the microbial ecology of the food product, the molecular mechanisms of the microbial inactivation by HPP, and the mode of action of the EO being used.