[intellij-idea] Adding Jar files to IntellijIdea classpath

I have created a new Intellij project. But I can't use the third party jars in my project. I have the jars in a directory structure as follows:
repository/commons-logging/commons-logging-1.0.4.jar
repository/commons-lang/commons-lang-2.1.jar
etc.

I want to add them just by pointing to the repository directory, but couldn't find any ways to add them. But when I am using classes or API from that jars, the editor can't resolve the classes. I have tried from Project Structure but can't configure it.

If, as I just encountered, you happen to have a jar file listed in the Project Structures->Libraries that is not in your classpath, the correct answer can be found by following the link given by @CrazyCoder above: Look here http://www.jetbrains.com/idea/webhelp/configuring-module-dependencies-and-libraries.html

This says that to add the jar file as a module dependency within the Project Structure dialog:

1. Open Project Structure
2. Select Modules, then click on the module for which you want the dependency
3. Choose the Dependencies tab
4. Click the '+' at the bottom of the page and choose the appropriate way to connect to the library file. If the jar file is already listed in Libraries, then select 'Library'.

On the Mac version I was getting the error when trying to run JSON-Clojure.json.clj, which is the script to export a database table to JSON. To get it to work I had to download the latest Clojure JAR from http://clojure.org/ and then right-click on PHPStorm app in the Finder and "Show Package Contents". Then go to Contents in there. Then open the lib folder, and see a bunch of .jar files. Copy the clojure-1.8.0.jar file from the unzipped archive I downloaded from clojure.org into the aforementioned lib folder inside the PHPStorm.app/Contents/lib. Restart the app. Now it freaking works.

EDIT: You also have to put the JSR-223 script engine into PHPStorm.app/Contents/lib. It can be built from https://github.com/ato/clojure-jsr223 or downloaded from https://www.dropbox.com/s/jg7s0c41t5ceu7o/clojure-jsr223-1.5.1.jar?dl=0 .

Go to File-> Project Structure-> Libraries and click green "+" to add the directory folder that has the JARs to CLASSPATH. Everything in that folder will be added to CLASSPATH.

Update:

It's 2018. It's a better idea to use a dependency manager like Maven and externalize your dependencies. Don't add JAR files to your project in a /lib folder anymore.