This research brief is the manual of the Historical Homicide Monitor (HHM), providing background information, general coding instructions, and the detailed Historical Homicide Monitor 2.0 codebook. The Historical Homicide Monitor: • Is intended for use in long-duration homicide research, enabling comparisons of homicide patterns and rates over long time spans and across study regions. • Transforms information from qualitative-textual sources into a standardized and mostly quantitative format. • Is source neutral: any type of qualitative source can be made commensurate with other sources by using the instrument. • Is designed to be compatible with the European Homicide Monitor standard (Granath et al. 2011), with identical, compatible and new variables. Compatible variables come with transformation syntax for full or enhanced mutual comparability. Data structure is compatible. • Has been developed to reflect the main theories of historical criminology while retaining flexibility in theoretical operationalizations. • Has been developed and tested in the Nordic area but can be applied anywhere. • Has been developed and tested in early modern and modern data but can be applied and improved for use in any historical period. • Requires standard procedures of historical source criticism and criminological validity assessments when used in research. The Historical Homicide Monitor was developed in the cross-national and interdisciplinary project “Nordic Homicide from Past to Present”, which brought together Nordic criminologists and historians. Funded by the Scandinavian Research Council for Criminology, the project used early modern court protocols from Denmark (1608–1622), Sweden (1640–1650) and Finland (1640–1699), 20th century criminal justice sources from Iceland (1900–1989), and contemporary homicide monitors from the same countries (2006–2017) to create a long-duration homicide research instrument. The Historical Homicide Monitor 2.0 manual is licensed under the Creative Commons Attribution-ShareAlike 4.0 license. The manual can be used and distributed freely in research and teaching; no permissions are required. The manual can be used in full or in part. To ensure comparability and data merger options, variable names and categories should not be changed.