My parents complain about each other to me, and I don't know what to do.

I think the hardest thing in this is that you love both of your parents, and coulnd't and wouldn't really choose a side. But there's an expectation towards you, to agree what one is saying, even when you don't even want to hear your parents complaning about each other. What makes this situation harder is the bitter identification of the fact: your parents are just like you - they fight and shout and think that they're right. But the hardest thing for you is to realise: They're no longer on each others' side, where they used to be.

Tell your parents that you don't want to be in the middle. Its not your responsibility to be the messenger.

In my experience, parents usually say to you what they're really trying to communicate to the other parent, or even just sounding it out for the first time with someone they trust and that knows the parent as much as they do. If it bothers you, why not ask them whether they think the other parent should hear this over you? Or explain to them that it doesn't make you feel comfortable to be put in the middle in such a way. It's better to be forthright with this kind of thing than be forced to carry too much in silence.

Respectfully let them know how you feel and how it's affecting you. Use "I" statements instead of "you" statements to avoid sounding offensive or finger-pointing. You have every right to love both parents; emphasize how you love both of them and think highly of them.

Have the parents sit down and talk to each other about their emotional or ideas they may have. Having them speak to one another may solve some conflicts or predicaments

If there's no way you can help solve their problems, just listen. I guess they both could use a hearing ear.

Be understanding but don't take sides. If they attempt to press you into taking a side, recommend that they speak with each other instead. If you find that they keep using you as an outlet and you're uncomfortable with this, tell them that you appreciate that they feel comfortable talking to you about these issues but that you aren't really in a position to help resolve the problems--they can only resolve it between themselves. There is no guarantee this will work but anything that can encourage them to share their complaints with each other instead of foisting them upon you is a positive step.

Tell them it's non of your business. Because it really isn't. Their relationship is something they need to sort out on their own and they should never put their children in the middle of all that.

I had to have a chat to my Mum about it, and explain that it hurt me. This was several years later and after I finally spoke to a councilor about my problems. If you can, talk to your parents. If you can't, talk to someone who can support you.