What is the reason for a red exclamation mark next to my project in Eclipse?

I have a red exclamation mark over my project name in Eclipse, looking like this - .

Does anyone know what this means and what I should I do about it?

There is a Problems view (try Window->Show View) which shows this kind of thing.

It's usually missing Jars (eg your project configuration references a jar that isn't there), and that kind of thing, in the case of JDT, but obviously these days Eclipse can be used in so many ways, it could be anything.

It means there is a problem with the build path in your project. If it is an android project then it mostly means the target value specified in project.properties file cannot be found. This can also be caused because of other kinds of built problems. But it is shown mostly for built problems only. See here for more details. It is about built error decorater seen in eclipse.

An extract from that page:

Build path problems are sometimes easy to miss among other problems in a project. The Package Explorer and Project Explorer views now show a new decorator on Java projects and working sets that contain build path errors:

The concrete errors can be seen in the Problems view, and if you open the view menu and select Group By > Java Problem Type, they all show up in the Build Path category:

I also had this problem several times and I could 'solve' it by just deleting the messages in the "Problems"-tab and building the project again. Select the project and click on the menu bar Project -> Clean... select your project again and okay.

There might be some deleted jar files already in build path. Right click on project. Properties->Java Build Path and removed deleted jar files from there.

There can be several reasons. Most of the times it may be some of the below reasons ,

1. You have deleted some of the .jar files from your /lib folder
2. You have added new .jar files
3. you have added new .jar files which may be conflict with others

So what to do is we have to resolve those missing / updating / newly_added jar files.

1. right click on the project and go to properties
2. Select Java Build Path
3. go to the Libraries tab
4. Remove the references of the jar files which you have removed already. There will be a red mark near them so you can identify them easily.
5. Add the references to the newly added .jar files by using Add JARs
6. Refresh the project

This will solve the problem if it's because one of the above reasons.