California Dept. of Consumer Affairs (DCA)
This agency licenses and regulates professionals such as doctors, nurses, dentists, optometrists, architects, court reporters, veterinarians, automotive repair technicians, accountants, barbers, and others.
(800) 952-5210 or (800) 344-9940

Contractors State License Board (CSLB)
This portion of DCA licenses and regulates contractors in the various construction trades, such as building, engineering, landscaping, plumbing, concrete, alarm installation, sanitation systems, swimming pools, roofing, and others. See complete list here.
(800) 321-CSLB (321-2752)

Better Business Bureau (BBB)
Their core services include:
• Business Reliability Reports
• Dispute Resolution
• Truth-in-Advertising
• Consumer & Business Education
• Charity Review
(916) 443-6843

California Attorney General (CAG)
The AG works to protect Californians and legitimate businesses in the state through the enforcement of consumer protection and fair competition laws. The Attorney General also issues consumer alerts and other materials to inform and educate consumers.
To file a complaint, call (800) 952-5225