A proof of claim is the form document a creditor must complete to be paid from the assets of a bankruptcy case. The proof of claim generally tells the bankruptcy trustee whether the claim is secured or unsecured, priority or nonpriority, and the amount of the claim. All creditors who want to be paid from the bankruptcy estate must file a proof of claim in Chapter 7, 12, and 13 bankruptcy cases—except in Chapter 7 no-asset cases, as there are no funds for distribution to creditors.

In California, as in all states, a proof of claim is a form that creditors must file in a bankruptcy case to assert their right to receive a payment from the debtor's bankruptcy estate. This form must detail the nature of the claim, specifying whether it is secured or unsecured, and if it holds priority status. Secured claims are backed by collateral, unsecured claims are not, and priority claims are those that the Bankruptcy Code has determined should be paid before other claims. Creditors are required to file a proof of claim in Chapter 7 (liquidation), Chapter 12 (family farmer or fisherman reorganization), and Chapter 13 (individual debt adjustment) bankruptcy cases if they wish to participate in any distribution of the bankruptcy estate's assets. However, in Chapter 7 cases that are deemed 'no-asset' cases—where the debtor has no nonexempt assets to distribute—creditors are not required to file proofs of claim because there will be no distribution. The proof of claim must be filed by a certain deadline, which is typically outlined in the notice of the bankruptcy case sent to creditors.