In 1929, the Scientific Crime Detection Laboratory was established in Chicago–the first of its kind in the United States. The purpose was to engage in practical use of scientific methods in the detection of crime. In 1930, the institute published its own periodical called The American Journal of Police Science. Applying the theory of boundary-work, this essay analyses how the novelty institute argued its legitimacy as a scientific establishment through the expansion of Police Science as a collective field of knowledge. The boundaries of Police Science expanded through certain patterns: the need for, and success of, science in solving crimes; its connection to the Northwestern University, engaging in education and research; by relating contested fields of knowledge to established sciences; through technological artefacts; by language demarcating “pseudo-science” from “real science”; and as an activity based on structure. Through professionalization, higher education and official accreditation the expert became science-by-proxy representing his field of knowledge in the court of law. Exclusion of pseudo-scientists was imperative to maintain and establish epistemological and scientific authority. Influenced by the “Progressives”, Police Science included reforms such as basic education for police officers; the removal of illegal and unscientific, but institutionalised, practices such as “third-degree” to regain the public‟s trust. In the conflict between the old-school and the new generation, Sherlock Holmes became a symbol used by both sides to discredit the opponent.