Estimation of postmortem interval (PMI) is very important in forensic investigations and provides valuable information for legal and criminal investigations. Traditional PMI estimation relies on visible postmortem changes such as rigor mortis, livor mortis, and algor mortis. These methods, however, can be imprecise and have high error rates due to the influence of various environmental and physiological factors. Recent advances in molecular biology have led to the development of powerful molecular methods for determining the postmortem interval (PMI) with greater accuracy. These techniques leverage advancements in biochemistry, molecular biology, and genetics to analyze changes at the molecular level that occur after death. Molecular approaches to determining time since death can include analysis of microbial succession, RNA degradation, protein decay, metabolomic profiling, and epigenetic modifications. The examination of these molecular markers enables forensic scientists to obtain a more precise estimate of the PMI, even in complex cases where traditional methods may be ineffective. This chapter explores current knowledge of the various methods that are employed in the estimation of time since death in forensic investigations.