In California, grand juries investigate matters they deem important to their communities, and then issue recommendations for resolving or improving matters. They're also required to inspect prisons within their county's boundaries.
But in most other states, grand juries are generally called on for criminal investigations. Grand juries are mandated by the constitutions of both the United States and California.
In the U.S. Constitution, grand juries are mentioned in the fifth amendment. In California's constitution, it's Article I, Section 23, and reads "One or more grand juries shall be drawn and summoned at least once a year in each county."
Jurors must be at least 18 years old, U.S. citizens, and residents of the county and state for at least one year. They must also have a good command of English.
Nevada is the only other state where grand juries function as watchdogs over county government.