If you run into problems using Drush with Drupal 8 you probably need to upgrade Drush.
I received this message - Error: Class 'Drupal\Core\Session\AccountInterface' not found, and found the instructions from http://docs.drush.org/en/master/install-alternative/ and http://docs.drush.org/en/master/install helpful.
1. Install Composer globally
$ curl -sS https://getcomposer.org/installer | php $ sudo mv composer.phar /usr/local/bin/composer
2. Add Composer's directory to your path by editing your .bash_profile or .zshrc
$ export PATH="$HOME/.composer/vendor/bin:$PATH"
3. Reload your terminal or just re-source the configuration you just added.
$ source $HOME/.bashrc
4. Now that you have Composer working, install Drush from the latest HEAD.
$ composer global require drush/drush:dev-master
Removing Conflicts from Existing Drush Dependencies
I ran into some problems due to previous Drush dependencies.
I had Drush 7 installed, and its dependencies caused failures when installing Drush 8 with Composer. I removed the conflicts with the following commands:
$ composer global remove drush/drush $ composer global remove package/package
Additional Drush Upgrade Instructions
Here are a few alternative installation instructions you may find useful: