Filth flies are major insect vectors responsible for the spread of human food-borne disease, especially those causing diarrheal illness. In Thailand, the three most common families of synanthropic flies are Muscidae (house flies), Sarcophagidae (flesh flies), and Calliphoridae (blow flies). Adult flies are not only a nuisance pest but also play an important role as mechanical transmitter of many pathogenic microorganism agents, such as bacteria, viruses, fungi, protozoa and helminthes. The larvae of flies can cause myiasis or infestation of the tissues and organs of live humans and animals. However, the immature stages of flies (eggs, larvae, and pupae) are useful for forensic entomology in criminal investigations, especially estimates of the postmortem interval (PMI). Consequently, a more detailed understanding of the biology of flies helps explain their role as carriers of disease and allows the public health planning for adequate control measures.