Carlsbad, California is located in San Diego County. Carlsbad is often referred to as being in "North County San Diego".

San Diego City is in San, Deigo County, California. Was not that a hard one?

Vista is a city in North San Diego county, California.

The counties that border San Diego County are: Orange County (northwest), Riverside County (north) and Imperial County (east) all located in California, United States. There is one county which could be called a county which is not called a county but is called a municipality. The municipality that borders San Diego County to the south is Tijuana Municipality located in the Baja California part of Baja California Pennisula in México.

The cities in California with the largest population are Los Angeles, San Diego, San Jose, San Francisco and Fresno. The counties in California with the largest population are Los Angeles County, San Diego County, Orange County, Riverside County and San Bernardino County.