Whoever controls the private key controls the bitcoin. When you leave coins on an exchange, the exchange is the custodian: it holds the keys, and you hold an IOU.
When custodians fail
- Mt. Gox, once the largest exchange, filed for bankruptcy protection in February 2014 after customer bitcoin went missing. Repayments are still ongoing; the trustee's deadline is now 31 October 2027.
- FTX collapsed in 2022. Its founder was sentenced to 25 years in 2024 for misusing over $8 billion of customer funds.
Bitcoin's answer
Bitcoin was designed so you need no "trusted third party" (white paper). With self-custody, your wallet holds your keys. No one can freeze, lend out or lose your coins but you.
The responsibility
Self-custody moves the risk to you. If you lose your seed phrase, no help desk can recover it. Start small, back up carefully, and consider a hardware wallet for larger amounts.
Takeaway
Exchanges are useful for buying. For saving, holding your own keys is what makes bitcoin yours.