Basil is used as a herb/spice and medical remedy all over the world. Various cultivars are been known to be rich in terpenes and terpenoids. However, the specific concentrations within leaves are not known, as typically only essential oils are analyzed. In order to investigate basil leaves directly, these were dried and terpenes and terpenoids were extracted with isooctane. Resulting extracts were subjected to chemical analyses. By using gas chromatography−mass spectrometry in combination with mass spectral database searches, we have identified 52 terpenes and terpenoids directly from leaf extracts. Furthermore, we employed 17 chemical standards to verify the correct identification of these substances and utilize these as standards in gas chromatography-flame ionization detection analyses to absolutely quantify these 17 terpenes and terpenoids from 11 different basil varieties for the first time. Here, terpene and terpenoid contents in the nanogram per milligram of leaf material range were observed. The most abundantly quantified substances proved to be β-myrcene, eucalyptol, linalool, eugenol, methyleugenol, and neophytadiene, while the determined concentrations varied significantly between investigated species/cultivars that might aid olfactory or medical applications in the future. Furthermore, principal component analyses were carried out with gas chromatography−mass spectrometry data. Here, 4 distinct groups of basil species/cultivars were identified and all 11 investigated basil varieties spanned these groups that were termed citrus, cinnamon/clove, spicy, and common. The successful separation into distinct groups in principal component analyses might lead to applications in food/drug fraud investigations in the future. Further insights into the phytochemical diversity of basil varieties have been obtained.