In law, commingling is a breach of trust in which a fiduciary mixes funds held in care for a client with his own funds, making it difficult to determine which funds belong to the fiduciary and which belong to the client. This raises particular concerns where the funds are invested, and gains or losses from the investments must be allocated. In such circumstances, the law usually presumes that any gains run to the client and any losses run to the fiduciary who is guilty of commingling. As one source puts it, "[i]n a pejorative sense, commingling is the special vice of fiduciaries (trustee, agents, lawyers, etc.) in failing to keep a beneficiary's money separate from the fiduciary's own money".

Commingling is particularly an issue in case of bankruptcy of the fiduciary. Funds held in care are not the fiduciary's property, and the client is not a creditor. So in case of bankruptcy, if the funds have been properly kept separate, they can easily be returned to the client. If, however, the funds have been commingled, the client is potentially subject to becoming entangled in the bankruptcy proceedings, and there may not be sufficient funds to pay the client back.