I will report on recent work with Liat Kessler on Hamiltonian circle actions on symplectic four-manifolds. Following work of Delzant and Karshon, Hamiltonian circle and 2-torus actions on any fixed simply connected symplectic four-manifold were characterized by Karshon, Kessler and Pinsonnault. What remains is to study the case of Hamiltonian actions on blowups of two-sphere-bundles over a Riemann surface of positive genus. These do not admit 2-torus actions. In joint work with Kessler, we have characterized Hamiltonian circle actions on them, up to (possibly non-equivariant) symplectomorphism. As a by-product, we provide an algorithm that determines the reduced form of a blowup form and which also provides a method for computing the Gromov width. If time permits, I will also discuss the circle equivariant cohomology of these manifolds, in terms of the fixed point set, which can include isolated fixed points and fixed surfaces.