In the United States, there are two main sources of criminal justice statistics, one based on reported crime, the other on surveys of the population:

(1) The Uniform Crime Report, administered by Federal Bureau of Investigation, compiles data of crimes reported to local police. The UCR was established in the early 1930s. Because it is based on local information, the UCR permits statistical analysis for local areas.  You can download some data from their site in spreadsheet format as well as pdf, which makes is easy for analysis. 

The accuracy of the UCR varies with the likelihood that people will report crimes to the police. The UCR is most accurate for murders, which are almost always reported or noticed, but may be less accurate for crimes that are not reported as often, such as rape or assault. In addition, for crimes such as theft, the UCR tends to underrepresent the frequency of offenses (because people do not always report minor theft), but over represent the value of thefts (because people are more likely to report major thefts). Long-term analyses of UCR data are also questionable because computers have permitted better police record keeping in the last 20 years. Therefore, increases in UCR crime rates partly result from improved recording of crimes by police. Still, the UCR is the standard & most widely cited method of measuring crime in the US.  A big advantage of this data is that it is available for each jurisdiction that reports to the FBI.

(2) The National Crime Victimization Survey, begun in 1973, is administered by US Census Bureau. This measure is a representative telephone sampling of approximately 40,000 households to determine how many people were victimized by one of seven crimes in past year. The crimes recorded are rape, robbery, assault, personal theft, household theft, burglary, and motor vehicle theft; the NCVS does not measure murder rates because the victims cannot be surveyed. The NCVS employs a national survey, so it cannot break down data by state or locality. Samuel Walker and most criminologists see the NCVS as more accurate than the UCR; people will acknowledge that they have been victims of a crime to an anonymous survey, even they did not report the crimes to police.

(3) A third method which is just beginning to be used is to collect data from hospital emergency rooms on crimes such as aggravated assault and spousal abuse.  This gets information on crimes not reported to the police. 

Determining rates of crime (generally, per 1000 people or 100,000 people) requires that the number of offenses (the numerator) be divided by an accurate count of the population (the denominator). The results of the decennial United States Census are conventionally used as the sources of population data.

[Adapted from Steven R. Donziger, ed., The Real War on Crime: The Report of the National Criminal Justice Commission (NY: Harper, 1996).]

The Bureau of Justice Statistics provides information on CJ statistical sources. as well as interesting time series graphs on CJ trends.  It also sponsors the Source Book of Criminal Justice Statistics.  Their Crime & Justice Online page provides links to data from states and cities.  You can also find data on specific jurisdictions in many of the other sources including the following:

They also offer other statistical reference products online--

State and Metropolitan Area Data Book --Comprehensive data for states and metropolitan areas

USA Counties --Over 5,000 data items for all counties

County and City Data Book -- Data for all counties and cities with a population of 25,000 or more

State & County QuickFacts  -- Quick, easy access to Census facts about people, business, and geography

MapStats -- Profiles with data from many Federal sources for your state, county, and city

Today, crime data are most often accessed on line, although paper publications and cd-roms are also available.  An advantage of on line sources is that the data are continuously updated.  Finding the most recent data is often important.  Finding data for small geographic jurisdictions is also a challenge.

A good place to look  for data is the Statistical Abstract of the United States, an annual volume available on paper and cd-rom, but more conveniently on line.  The files open in pdf format, however, not in spreadsheet form.  Section 5:  Law Enforcement, Courts and Prisons, provides the CJ data.  It gives the sources for much of the data in other US government publications which are available online or in libraries.