The science of bloodstain-pattern analysis (BPA) applies scientific knowledge from other fields to solve crime-related problems. BPA draws on biology, chemistry, math, and physics. The article first outlines general precautions that should be taken by investigators and crime-scene technicians in handling blood and other biological fluids at a crime scene. It then discusses how bloodstain evidence at a crime scene can provide important information on the nature and sequence of crime-related events. This is followed by an explanation of Locard's Principle. This principle holds that the perpetrator will take away traces of the victim and the crime scene; the victim will retain traces of the perpetrator and may leave traces of himself or herself on the perpetrator; and the perpetrator will leave traces of himself or herself at the crime scene in many ways. This principle is applied in the retrieval and evaluation of bloodstain pattern evidence. The next section of the article focuses on how to process the crime scene for the retrieval of bloodstain pattern evidence, followed by sections on bloodstain classification. The latter discussion addresses classification of stains by evidence of velocity; by taxonomy (a set of laws or principles for classification); and by the physical features of size, shape, location, concentration, and distribution. Classification sets the stage for the analyst to define a source event for any given stain. The article concludes with discussions of classic bloodstain patterns and their documentation. Photographs and a graphic illustration present examples of the principles and topics discussed.