You can install the newest bootstrap (currently v.5) in a few different ways:

1. (The easiest way) By downloading a .zip package here
2. Bo downlading a source file here
3. By using CDN here
4. By using package managers (npm, yarn, RubyGems, composer, NuGet) here

You can also download for free an enhanced version of Bootstrap, called MDBootstrap.

Download MDBootstrap

Detailed documentation and more examples you can find in our documentation

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Previous versions of Bootstrap:

1. Bootstrap 4
2. Bootstrap 3
3. Bootstrap 2