A useful tool to study a 3-manifold is the space of the
representations of its fundamental group, a.k.a. the 3-manifold group, into
a Lie group. Any 3-manifold can be decomposed as the union of two
handlebodies. Thus, representations of the 3-manifold group into a Lie group
can be obtained by intersecting representation varieties of the two
handlebodies. Casson utilized this observation to define his celebrated
invariant. Later Taubes introduced an alternative approach to define Casson
invariant using more geometric objects. By building on Taubes' work, Floer
refined Casson invariant into a graded vector space whose Euler
characteristic is twice the Casson invariant.  The Atiyah-Floer conjecture
states that Casson's original approach can be also used to define a graded
vector space and the resulting invariant of 3-manifolds is isomorphic to
Floer's theory. In this talk, after giving some background, I will give an
exposition of what is known about the Atiyah-Floer conjecture and discuss
some recent progress, which is based on a joint work with Kenji Fukaya and
Maksim Lipyanskyi. I will only assume a basic background in algebraic
topology and geometry.