Superconducting cavities have been in operation in accelerators for 25
years. In the last decade many installations in storage rings and lin
acs have been completed. Meanwhile, nearly 1 km of active cavity lengt
h is in operation in accelerators. Large-scale applications of superco
nducting radiofrequency systems are planned for future e(+)e(-) linear
colliders and proton linacs. Superconducting cavities have been prove
d to operate at higher gradient, lower AC power demand and more favour
able beam dynamics conditions than comparable normal conducting resona
tors. The performance of the best single-cell cavities:comes close to
the intrinsic limitation of the superconducting material. Complete mul
ticell structures with all auxiliaries (couplers, tuner, etc) lag behi
nd in performance because of their complexity. In this paper, an overv
iew of accelerators with superconducting cavities is given. Limitation
s of superconducting performance are described and research and develo
pment efforts towards understanding and curing these effects are discu
ssed in detail. Fundamentals of superconductivity and radiofrequency c
avity design are briefly explained.