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The cryogenic current comparator (CCC), first demonstrated nearly 40 years ago, has become a key component of electrical metrology. It utilises a superconducting screen to achieve very high ratio accuracy and hence has found many applications where the electrical units need to be scaled over decade values. It has been deployed over a wide range of currents from 100 A to 1 pA and has been used to verify the accuracy of electrical quantum effects such as the quantised Hall effect. This study is a review of the theory, design principles and most common applications of the CCC. In addition to the summary of its use in top-level electrical metrology, some recent developments such as the use of high temperature superconducting materials and applications outside the realisation of electrical units are described.