The Open Brethren, sometimes called Christian Brethren, are a group of Evangelical Christian churches that arose in the late 1820s as part of the Assembly Movement within the Plymouth Brethren tradition. They originated in Ireland before spreading throughout the British Isles, and today they have an estimated 26,000 assemblies worldwide.

The Plymouth Brethren or Assemblies of Brethren are a low church and Nonconformist Christian movement whose history can be traced back to Dublin, Ireland, in the mid to late 1820s, where it originated from Anglicanism. The group emphasizes sola scriptura, the belief that the Bible is the only authority for church doctrine and practice. Plymouth Brethren generally see themselves as a network of like-minded free churches, not as a Christian denomination.