[en] Defects in crystalline and amorphous SiC are studied by different positron annihilation methods. Small vacancy clusters are present in 'as grown' crystalline 6H bulk SiC and are stable up to high temperatures. Mono- and divacancies are created by electron irradiation. These defects are observed in the neutral charge state and do not act as carrier traps. Around 100 C a large fraction of the vacancies anneal by Frenkel pair recombination. The vacancies become first mobile at 700 C and get trapped by doping and transition metal impurities. Annealing between 1200 and 1400 C removes these trapped vacancies. The composition and optical properties of amorphous hydrogenated SiC films grown by a CVD process can be controlled by the flow rate of the gas source. Open volume defects are formed during the film growth due to the incomplete dissociation of the source molecules. These defects are interconnected throughout the amorphous network via channels and can trap gases in the large internal surface area. The gases can be desorbed from the internal surfaces by heat treatments and leave behind an increased areal density of defects in the sample. A nano-crystalline structure grows upon annealing and irreversible structural changes take place, when breaking of Si-H and C-H bonds and formation of additional Si-C and C-C bonds lead to film densification. (author)