Bloodstain pattern analysis has been used in criminal investigations for more than a 100 years. Analysis has been made faster through modern methods of measuring bloodstain patterns. However, since the beginning the basic principles of the analysis and the conclusions drawn have not changed. Bloodstain pattern analysis provides important forensic information about the crime under investigation; it tells what happened. Bloodstain patterns occur in several distinct categories, each revealing a piece of the crime scene puzzle. There are seven bloodstain categories: (1) passive bloodstain; (2) projected bloodstains; (3) impact spatter; (4) cast-off bloodstains; (5) arterial gush or spurt bloodstains; (6) wipe bloodstain patterns; and (7) transfer bloodstains. Crime scene investigation (CSI) technicians make a series of different measurements on the bloodstains at a crime scene and the data are used to reconstruct what happened during the commission of a crime scene. However, bloodstains can be widespread at a crime scene and present in areas not immediately obvious to the first officers on the scene. So, upon arrival at the crime scene, the responding officers should secure the site and ensure the crime is not still in progress. Every effort must be made to preserve the crime scene until the bloodstain evidence has been investigated. Bloodstain pattern analysis is a powerful tool used in solving violent crimes and must be performed by well-trained individuals. It is one of the most effective methods of reconstructing crime scenes available to forensic analysts.