Today I noticed that several of my seedlings have yellow, brown leaves, what could be the problem? And how do I stop it? To much water? Not enough water?

Not sure if these are the same, but I'm in the midst of hardening off my seedlings, and they have all lost a few leaves. I think it's a normal thing. They are by no means dying, and continue to grow and put out new leaves. I'm new to all this too, so hope others with actual experience will chime in.

Yep, set mine out to harden off and the same thing happened, but only to those that had really long leaves. Some were dormants that were just waking up and their leaves stayed green.

I think the older leaves just died and are rapidly being replaced by new leaves. Nothing to worry about.