Essential oils represent a good alternative to chemical biocides as they have antiseptic, antibacterial and antifungal properties, inhibiting the formation and proliferation of biofilms which can occur on outdoor and indoor surfaces of ancient and modern artworks. In this review, we illustrate how their antimicrobial properties, known since antiquity in phytotherapy, have been studied and tested for conservation purposes since the 1970s. In vitro tests on a wide range of plant extracts and in situ applications of specific volatile compounds have shown selective antibacterial and antifungal properties after the individual action of pure components or as the synergic effects of pre-determined mixtures. The review emphasizes the broad spectrum of materials—organic and inorganic—that essential oils can be applied to as biocides and finally emphasizes how the demand for commercial solutions has rapidly grown in bioconservation. The review demonstrates how research on the subject has been powerfully boosted by the ecofriendly and harmless character of essential oil applications, which makes them one of the most sustainable options in heritage conservation nowadays. The review elucidates how research is developing novel solutions for the application of EO blends—like encapsulation and microemulsions—and their optimization in commercial products for heritage conservation.