SN1998bw is a peculiar type Ic supernova. Its peak brightness is almost ten times greater than typical type Ic in optical and even 10-100 times brighter than other radio bright type Ib/c supernovae in radio bands. The extremely broad features in the optical spectra indicate high velocities of ejected matter char 126 3 × 10(4) km s-1 which, combined with the breadth of light curves, lead to an estimated kinetic energy of explosion of char 1262-5 × 10(52) erg under the assumption of spherical symmetry. This class of hyperenergetic supernovae is termed "hypernova". There are some observational indications of possible asymmetry, which might reduce this energy requirement. However, it is still likely that SN1998bw is a hypernova. SN1998bw is the first supernova for which a possible link to a gamma-ray burst was suggested due to its angular and time coincidence with GRB980425. The high radio luminosity and fast evolution of the radio light curves suggest the existence of a relativistic blast wave in SN1998bw, which might be related to the gamma-ray burst directly or indirectly. Several other several hypernova candidates have been discovered or reinterpreted since SN1998bw. The unusual properties of these objects may require reconsideration of theories of stellar explosion mechanisms.