Terpenoids constitute the largest and most diverse class of natural products. They are important factors for aroma and flavor, and their synthesis is basically done from two compounds: isopentenyl diphosphate and dimethylallyl diphosphate. Isopentenyl diphosphate is synthesized through two different pathways, one that occurs in the cytoplasm and one in the plastid. With the sequencing of ESTs from citrus, we were able to perform in silico analyses on the pathways that lead to the synthesis of terpenes as well as on the terpene synthases present in sweet orange. Moreover, expression analysis using real-time qPCR was performed to verify the expression pattern of a terpene synthase in plants. The results show that all the pathways for isopentenyl diphosphate are present in citrus and a high expression of terpene synthases seems to have an important role in the constitution of the essential oils of citrus.