The transfer of hairs and fibers and their discovery as trace evidence can be critical in linking a suspect to a victim or to a crime scene. Knowing how hairs and fibers can be transferred and which factors affect the significance of a hair or fiber match are important concerns of crime scene technicians, laboratory examiners, investigators, and prosecutors. Regarding hair, the types of hair recovered and the condition and number of hairs found all pertain to their value as evidence in a criminal investigation, comparison of the microscopic characteristics of questioned hairs to known hair samples assists in determining whether a transfer may have occurred. This article discusses the examination of human hairs in the forensic laboratory using light microscopy, hair anatomy and growth, and the analysis of animal hairs and human hairs obtained from various parts of the body. The significance of hair evidence is discussed in detail. Regarding fiber evidence, it is significant because textile fibers can be exchanged between two individuals, between an individual and an object, and between two objects. Fibers from the suspect's clothing may be found at the crime scene or on the victim, and fibers from the crime scene or the victim may be found on the suspect. This article discusses the nature of fiber evidence, the types and sources of fabrics, fiber transfer and persistence, and assigning significance to fiber evidence. The article concludes with a section on the crime-scene search for hair and fiber evidence, the collection of hair and fiber evidence, evidence handling procedures, evidence processing in the FBI Laboratory, and examination conclusions.