A criminalist is a person with a background in science, typically having at least a bachelor degree in an area such as chemistry, biology, forensic science, or criminalistics. Some criminalists have degrees in other, similarly related areas. Many criminalists have advanced degrees.

With the above scientific background and additional training given by his/her employer (either a government or private laboratory) a criminalist applies scientific methods and techniques to examine and analyze evidentiary items and testifies in court as to his or her findings.

Criminalistics is one of many divisions in the field of forensic science. Forensic science includes forensic pathology, odontology, entomology, engineering, criminology, and other disciplines. All of these are specialized sections in forensic science. Criminalists use techniques learned in chemistry, molecular biology, geology, and other scientific disciplines to investigate and solve crimes. Criminalistics should not be confused with the field of criminology. Criminologists are sociologists, psychologists, and others who study the causes and effects of crime on society.

For the criminalist, crime scene investigation involves the recognition, documentation, collection, preservation, and interpretation of physical evidence which may be as big as a truck or as small as a diatom or pollen grain. Recognition of items out of place, articles improperly located or items added to the crime scene are an important part of crime scene processing. The criminalist collects, preserves, and makes interpretations about the evidence and their relation to the series of events resulting at the crime scene.

The criminalist brings evidence back to the laboratory where examinations will be conducted. Interpretations are made about the relevance of a particular item from the crime scene by associating particular items of evidence to specific sources and reconstructing the crime scene. This means not only associating a suspect with a scene but also the telling of a story about what transpired before, during and after the crime. The criminalist must draw on a wide spectrum of scientific knowledge including chemistry, biology, genetics, molecular biology, physics, statistics and a working knowledge of civil and criminal law. Applying this knowledge, criminalist will associate and identify evidence, interpret the results, reconstruct the crime scene, and write a report summarizing the findings.

Finally, the criminalist testifies in courts of law, teaching the judge and jury about the conclusions reached in the laboratory.