Post-mortems are an essential part of the investigative process after someone has died in suspicious circumstances, usually performed to establish cause of death. Definitively proving time of death later is extremely difficult. By using blowflies and sometimes other insects, forensic entomologists can provide an estimated window of time in which someone is likely to have died. This is calculated by estimating the amount of time since eggs were first laid, which approximates (sometimes quite closely) the time of death. Such insect derived time is known as the minimum post-mortem interval (PMImin).