From 1970 to 1979, the number of complaints against the police has risen significantly, although the population of the Metropolitan Police district has decreased during the decade. The proportion of complaints withdrawn has increased only slightly, but the new category of 'not proceeded with' complaints has grown considerably since its introduction in 1977. Despite the fact that each complaint is supposed to be considered on its merits, the likelihood of substantiation was found to vary according to the type of complaint. Assault complaints, which are on the increase, were much less likely to be substantiated than less serious complaints. Complainants under arrest, those reported for an offense, and those having a criminal record made up an increasing proportion of complainants; yet, they were much less likely to have their complaints substantiated. Blacks, and to some extent Asians, are making up a large and increasing proportion of complainants. Very few black complaints were substantiated, however. Black complainants were also found to be more likely to make complaints of assault than whites or Asians. Further, blacks were more likely to be under arrest, to have been reported for an offense, or to have a criminal record. These characteristics would lead one to expect a low substantiation rate for black complainants. There were no substantial differences in the circumstances giving rise to assault complaints from members of different ethnic groups. Future research should examine why so few black complaints are substantiated and why assault complaints are unlikely to be substantiated. Tabular data and 108 references are provided. (Author summary modified)