  In this paper, I will present the recent progress on circular-ribbon flares
(CRFs) and their related activities, including coronal jets, filaments, CMEs,
radio bursts, coronal dimmings, and coronal loop oscillations. Owing to the
prevalence of 3D magnetic null points and the corresponding fan-spine topology
in the solar atmosphere, CRFs are regularly observed in UV, EUV, and H$\alpha$
passbands. Spine reconnection and fan reconnection around the null points are
predominantly responsible for the energy release and subsequent particle
acceleration. Slipping reconnection at QSLs may explain the sequential
brightening or rapid degradation of the circular ribbons. Periodic or
quasi-periodic acceleration and precipitation of nonthermal particles in the
chromosphere produce observed QPPs of CRFs in multiple altitudes as well as
wavelengths. Like two-ribbon flares, the injected high-energy particles result
in explosive evaporation in circular and inner ribbons, which is characterized
by simultaneous blueshifts in the coronal emission lines and redshifts in the
chromospheric emission lines. Homologous CRFs residing in the same active
region present similar morphology, evolution, and energy partition. The
peculiar topology of CRFs with closed outer spines facilitates remote
brightenings and EUV late phases, which are uncommon in two-ribbon flares.
Besides, CRFs are often accompanied by coronal jets, type III radio bursts,
CMEs, shock waves, coronal dimmings, and kink oscillations in coronal loops and
filaments. Magnetohydrodynamics numerical simulations are very helpful to
understand the key problems that are still unclear up to now. Multiwavelength
and multipoint observations with state-of-the-art instruments are enormously
desired to make a breakthrough.
