When it comes to negotiation, sometimes you have to just walk away.

When a negotiation is going well, and you and your adversary have reached a win-win (or at least a win-not lose) deal, it’s obvious when to stop negotiating. But when a negotiation turns tense or difficult, how do you know when to push through and keep going, or stop and move on? Here are some signs it’s time to call it quits.

1. It’s just not fair!”

It’s human nature for negotiators to determine their preferred outcome, then justify it based on fairness. A recent blog post from the Harvard Law School Program on Negotiation gives a great example: the Winklevoss twins’ case of “settler’s remorse” after receiving a $65 million settlement from Facebook. Arguing that Facebook had cheated them out of hundreds of millions of dollars, the twins waged an expensive and time-consuming battle based on dubious legal grounds to undo the settlement, claiming to be motivated not by money but by fairness.

The alleged pursuit of fairness can be a slippery slope. As the Harvard blog points out, say you’re splitting up with your business partner. Of course, you feel that you deserve a bigger slice of the pie, because you invested more money. But your partner feels that she deserves a bigger slice – because she invested more time. Our natural egocentrism can destroy a negotiation. If the negotiation has hit a wall because you or your adversary are claiming unfairness, take a step back. Ask a colleague or mentor for an objective reality check. It may be time to consider your opponent’s perspective more carefully – or move on.

2. You’re negotiating with the devil (or so you think).

Disputes can drag out over long periods of time – lasting months or even years. With each passing day, frustrations rise, impatience grows, and it becomes easier for the parties to stop seeing the humanity in each other and instead start to demonize each other. Ask yourself honestly: are you continuing to fight it out because you think you can get a better outcome, or because you want to stick it to your opponent? There’s tenacity, and then there’s obsession. In this situation, it may be helpful to ask a trusted advisor or colleague for an honest assessment about whether this fight it still worth pursuing.

3. No one’s budging.

Sometimes in a tense negotiation, no matter how much you prepared beforehand or how carefully you’ve determined your BATNA, a deadlock occurs and the parties simply cannot reach an agreement. When this happens, take some time to reconsider your own position and make sure you are actively listening to the other side’s concerns. (See some other tips here.) The deadlock may be a temporary setback that can be overcome. Still no movement? It may be time to walk away.

These difficult scenarios demonstrate why success in negotiation depends so strongly on your understanding of the psychological principles at play across the table. You owe it to yourself to prepare thoroughly and to keep in mind that a final resolution might not occur in the first round of negotiations. Calling it quits is a last resort, not a first.