  Thurston showed that the fundamental group of a close atoroidal 3-manifold
admitting a co-oriented taut foliation acts faithfully on the circle by
orientation-preserving homeomorphisms. This action on the circle is called a
universal circle action due to its rich information. In this article, we first
review Thurston's theory of universal circles and follow-up work of other
authors. We note that the universal circle action of a 3-manifold group always
admits an invariant lamination. A group acting on the circle with an invariant
lamination is called a laminar group. In the second half of the paper, we
discuss the theory of laminar groups and prove some interesting properties of
laminar groups under various conditions.
