Throughout the years numerous investigations concerning the inhibition of microorganisms by spices, herbs, their extracts, essential oils and various constituents have been reported. Many of these materials possess significant antimicrobial activity, which in many cases is due primarily to a particular constituent. Interpretation and comparison of results of various studies is complicated by variations in the methodology used for the determination of antimicrobial activity. The antimicrobial activity varies depending on the microorganism, the spice or herb and the test medium. These and other factors are examined in the light of their effect on the outcome of the test method.