Recent forensic advances provide real hope that old crimes may now be solved.
Since teenager Hayley Dodd vanished 14 years ago, there have been big developments in the science to find and analyse DNA, fingerprints, blood and other forensic evidence.
Though improved DNA analysis is the most common technique used for cold cases, police check whether other technology - including lasers to examine bullet fragments and glass or chemicals to find hidden fingerprints or substances - can provide breakthroughs.
Police have identified possible offenders and new leads in a number of unsolved crimes after retesting exhibits.
But they will not reveal details of the breakthroughs because the investigations are continuing.
They believe the benefits of better DNA analysis, which they have used since early 2012, may not be fully known for years until stored evidence is retested.
The technique tests 21 DNA markers instead of 10, increasing the chances of getting a profile from tiny samples or ones degraded by weather or over time, such as on clothing.
Police also now have technology to separate mixed DNA samples to identify sources.