This chapter focuses on a small number of estimator variables likely to affect witness testimony, before exploring investigative interviewing as a key system variable. The level of stress or fear a witness experiences during encoding, and the effect that such emotional reactions will have on subsequent memory performance, varies according to the nature of the incident and the individual witness. The provision of investigative interview training for more specialised officers is also limited – with worrying knock-on effects. The new retrieval techniques and mnemonics outlined above represent a promising step towards developing effective methods of incorporating witness-led cueing techniques into investigative interviewing contexts. The burgeoning academic interest in forensic and investigative psychology has generated increased opportunities for police investigators to harness this growing area of knowledge, to increase their own understanding of science-based practice, and in doing so, enhance their skills.