Mattermost is an open source, real-time, self-hosted chat, and collaboration system. It is frequently described as an alternative to Slack that can be run on one's own infrastructure. Similar to Slack, users access a Mattermost server either from a browser, a dedicated desktop chat client, or one of several mobile applications. Mattermost channels support embedded images and files. Channels are grouped into teams and are managed by team admins. Membership within a channel can be public, restricted to users from specific email providers, or by invitation only. A number of bots and integrations are available for Mattermost and more can be created with the Mattermost REST API. Detailed information on how to do so is available in their developer documentation. In the administrator's guide, users can find instructions for installing the Mattermost server on Ubuntu, Debian, Red Hat Enterprise Linux, and CentOS, among others. Mattermost can also be deployed to a number of cloud providers using Bitnami. In addition to the free, open-source release of Mattermost, an enterprise version that includes additional features for large organizations (single sign-on, multi-factor authentication, etc.) is also available for purchase.