android.support.v7.widget.CardView could not be found (Android Studio 1.1 Preview)

You should add it to your build.gradle:

dependencies {
    ...
    implementation 'com.android.support:cardview-v7:21.+'
    implementation 'com.android.support:recyclerview-v7:21.+'
    ...
}

Writing my answer to provide:

1) updated version for compileSdkVersion 27.

2) usage of implementation instead of compile - What's the difference between implementation and compile in Gradle?.

3) a complete version number to heed the Avoid using + in version numbers; can lead to unpredictable and unrepeatable builds.., warning shown by Android Studio.

4) a visual for location of build.gradle (Module:app).

5) And because I was not able to write a comment below @Eun's answer.

You need to paste implementation 'com.android.support:cardview-v7:27.1.1' in build.gradle [Module:app] located at, for example: MyFirstApplication -> app -> build.gradle

Paste in following dependencies clause:

dependencies {
   implementation fileTree(dir: 'libs', include: ['*.jar'])
   ...
   implementation 'com.android.support:appcompat-v7:27.1.1'


   implementation 'com.android.support:cardview-v7:27.1.1'


   ...
}

To open from Android studio itself: (From left side) 1:Project -> Select Android view -> Gradle Scripts -> build.gradle (Module: app)

That is:
(If the image below is not shown You can find the image here.

Using Android Studio 3.1.4

Edit: Updated version numbers.

With the release of Android 9.0 (API level 28) there is a new version of the support library called AndroidX

And the support library will be discontinued...

Add the following to buld.gradle:app

dependencies {
    ...
    implementation 'androidx.recyclerview:recyclerview:1.0.0'
    implementation 'androidx.cardview:cardview:1.0.0'
    ...
}