Scientific Evidence

The scientific examination of physical evidence involves the application of the natural sciences to matters of law and includes a variety of disciplines. Evidence examinations in the forensic science laboratory employ the principles and methods of all the traditional sciences, including physics, chemistry, and biology. A forensic scientist's work is governed by Locard's principle, which states that “every contact leaves a trace.” This trace—a fiber or a hair, for instance—can be scientifically analyzed and compared to something found at a crime scene and may link a suspect to a crime. The duties of a forensic scientist include the laboratory examination of items recovered from a crime scene as well as examination of the scene of a crime. The scientist's conclusions are often presented as a report or statement that is subsequently given as evidence in court. As a result, appearing in court in order to present evidence and be subjected to cross-examination is an essential part of a forensic scientist's job.

The role of forensic science in the criminal justice system has gained notoriety in the past decade. Prominent court cases such as the O. J. Simpson and William Kennedy Smith trials have hinged on scientific evidence, and television shows such as Medical Detectives and C.S.I. have popularized the field. Although the general public is now more familiar with the capabilities of forensic scientists, some police professionals are still unaware of proper evidence collection guidelines or of all the capabilities the forensic laboratory has in examining scientific evidence. Without this knowledge, poor collection techniques may ruin otherwise viable physical evidence. Probative evidence may also be missed during evidence collection.