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The progress which has been made in recent years in analyzing and understanding the electromagnetic properties of helical structures is reviewed here. After a brief account of work prior to about 1950, the results obtained from an analysis of the sheath helix model are summarized. The more physically realistic tape helix model and its characteristics-forbidden regions, space harmonics, etc.-are discussed. Recent extensions to the analysis of the sheath helix model including the effect of radially stratified mediums, both isotropic and anisotropic, are noted. Also, additional work using the tape or finite wire helix model is reviewed. Finally, the recent investigations of filter helixes and contrawound helixes are discussed.