The research undertaken in this study has advanced the ability of an investigator to estimate postmortem interval (PMI) derived from blow fly evidence by providing new tools, tools that significantly increase the accuracy and precision of age predictions. The basic theory upon which the entire project was based, that gene expression can provide more precise age estimates, was established, and was most successful in developmental stages that are currently the most difficult to assess. At the same time, laboratory rearing conditions for blow flies that most accurately mimic those on a cadaver were generated, creating a standardized operating procedure that helps meet the tenets of Daubert. Likewise, the modeling of larval growth (length and weight), along with gene expression, allows for confidence intervals and error estimates to be produced. In total, the works performed have produced a substantial leap forward in using entomological data to more accurately estimate time since death. Blow fly evidence can be useful in estimating PMI during death investigations. This is due largely to the reliable development of blow flies and their fondness for colonizing remains within hours of death. Investigators can utilize blow fly evidence as a biological clock, using the evidentiary flies to backtrack to the time that remains were colonized. This research study, supported by the U.S. Department of Justice, National Institute of Justice (NIJ), utilized gene expression information in age estimates of the green bottle fly. This species was chosen because it is forensically useful and globally distributed. Tables, figures, and appendixes