Background: Estimation of postmortem interval is considered one of the most important issues in forensic medicine. It aids the police investigation and help to know the time of the crime and its relation to the assailant. It also important in some civil problems as inheritance. There are multiple methods that used for estimaton of PMI as: rate of cooling after death, detection of supravital reactions, rate of development of hypostasis, rigor mortis, life cycle of insect and the decomposition changes that occur after death. Also usage of body fluids as chemical method for estimation of time passed since death shows great advances. Recently application of DNA and RNA in this field has done aiming at more accurate estimation of PMI.
Aim of the work: Introducing and establishing the basic knowledge about Postmortem Interval estimation and focusing on new approaches for postmortem interval estimation.
Conclusion: PMI estimation is the time between time of death and time of identification of corpse. It has a role in forensic medicine specially in crimes. It helps to aid the police in their investigation and reduce the number of assailant. PMI estimation is still depend on early and late postmortem changes as algor mortis, rigor mortis, hypostasis and putrefaction. But recently there are new methods as detection of new biomarkers, using of microscopic changes that occur after death in multiple organs with success of these trials was noted and a correlation between them and PMI was found. DNA and RNA also participate in estimating time since death by detection the rate of degradation and its relation with PMI.  
Definition of postmortem interval (PMI):
Postmortem interval (PMI) means the time passed since death. Estimation of PMI is done through information that depend on estimation of sets of time: Time A  and  Time B . Time A is the time when the decent was last known to be alive, while time B is the time when the decent was found dead. The time since death is presented certainly between these times (between death and examination of the dead body) based on different factors as algor mortis ,rigor mortis , postmortem lividity and decomposition (Prahlow, 2010).
There are multiple early and late postmortem changes that may aid in PMI estimation