You should consider surrendering in blackjack when you believe your hand has a significantly low chance of winning against the dealer's upcard. This decision is often optimal when you have a weak hand, such as a total of 15 or 16, and the dealer shows a strong card, like a 9, 10, or ace. Surrendering can help minimize losses in situations where hitting or standing is unlikely to result in a favorable outcome. It's a strategic move to cut your losses when the odds are not in your favor.