Guest editor's introduction to this special issue on investigations Despite the fact that law enforcement agencies in the USA dedicate considerable resources to investigations, their level of clearing serious crimes is low compared to previous years and to other industrialized Western democracies (Alexander and Wellford, 2017). In 2019, clearance rates in the USA were as follows: 61% for murders and nonnegligent manslaughters; 33% for rapes; 31% for robberies; 52% for aggravated assaults; 14% for burglaries; 18% for larcenythefts and 14% for auto thefts (Federal Bureau of Investigation, 2020). Stated differently, out of 7,308,764 serious crimes reported to police in that year, 5,727,987 were not cleared (78%). This does not include the large number of crimes other than homicide that are not reported to police. By any measure law enforcement agencies in total are not doing as well as we should expect in solving crimes and bringing alleged offenders to consideration by our court systems [1]. Looked at over time, this situation is even worse. For example, we estimate that since 1980 there are 260,000 homicides and 15,000,000 aggravated assaults reported to police that have not been cleared [2].Research since the 1990s and that presented in this special issue have focused on explaining why there are variations in clearances across cases and agencies. Most of this work has been on homicide cases. That work has informed the funding by the Bureau of Justice Assistance of the research based technical assistance provided by the Police Executive Research Forum and the National Police Foundation to agencies that wanted to improve homicide clearances [3]. This volume provides new work on homicide clearances but also expands the approach derived from that literature to other offenses, especially physical and sexual assaults.Prior to the 1990s, the common wisdom in police studies was that police had little to do with crime clearance. The studies by RAND in the 1970s were interpreted by many (inaccurately as we explain in our paper in this volume) as finding that police did little to contribute to the clearance of crime. Prominent commentators (e.g. Bayley, 1994) went so far as to conclude that the police played no role in clearances. Some research prior to the 1990s that seemed to find a stronger role for police in crime resolution in fact argued that police should seek to identify the cases they could solve and ignore the rest (c.f., Eck, 1983); a position that assumes case characteristics largely explain clearance and is only slightly different from the position that police do not matter. Research since then has, at least for homicides, found that police can through organization, resource, personnel, training and investigative practices improve clearances of all types of homicide (