From 1990 to 2014, personal income in California grew fairly consistently, with limited volatility. On the other hand, California's personal income tax (PIT) base was much more volatile. This is because (1) some of the more stable pieces of personal income are not taxed under California's PIT and (2) the PIT tax base includes capital gains, which are extremely volatile and are not counted as part of personal income in federal statistics. This brief examines the volatility of the PIT tax base, one important element of the PIT's overall volatility in California. (This brief does not focus on other reasons for PIT volatility, such as California's PIT rate structure, in which high-income Californians pay a bigger fraction of their income than lower- and middle-income Californians.)