Forensic evidence and the way it was gathered came under close scrutiny in the trial of Stephen Lawrence's killers.
Techniques and practices have developed significantly since 1993 and are now a key part of police investigations from the start.
The basic principle is "every contact leaves a trace" and specks of blood, fibres or fingerprints become silent witnesses against the criminal.
Here, we examine forensic science from crime scene to courtroom.
I . CRIME SCENE
Preserving the scene
Uniformed officers are usually the first at a crime scene.
They are trained to cordon off anything which may potentially be relevant - be it a phone box, a car or a door which an offender may have touched.
An outer cordon excludes anyone but police officers.
An inner cordon is where the main forensic evidence is expected to be found.
Crime scene manager
A crime scene manager has to try to "stop the clock" to ensure as little is changed as possible.
It is his, or her, job to establish the precise area of the scene as well as a "common approach path" to the body or key bits of interest.
The officer will also check who has had access, such as police or ambulance crews, in case footprints need eliminating.
Common approach path
The CAP is often made up of metal stepping stone-style plates which give crime scene investigators access to the scene while minimising contamination of possible evidence.
Anyone entering or leaving the crime scene must take this route and their presence is logged.
Physical evidence
Physical evidence such as a knife, cigarette packet or bullet casings are photographed or sketched in situ.
They are logged, packaged and labelled before being sent for forensic examination at the laboratory.
The body
In a murder case, a body may remain in place for several hours before it is removed from the scene to allow all the necessary tests to be carried out.
Pathologists may be asked to visit the scene so they can put the body into context when they come to carry out the post-mortem examination.
II - LABORATORY
From the painstaking work at the crime scene, forensic examination moves to the lab.
Here, a scientist from the Metropolitan Police North West fingerprint lab talks us through a range of optical, physical and chemical techniques used to detect and enhance hidden marks.
Depending on whether the item is porous or non-porous, a range of treatments are used to obtain a visible representation of a finger mark which can then be photographed and sent for identification.
Finger marks linked to the scene are sent to the national fingerprint bureau at New Scotland Yard for identification against the national database.
Here, a fingerprint expert explains how suspects can be matched to marks, and vice versa.
