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An exposition of some of the past, present and future activity of the U. S. Army Signal Research and Development Laboratory in the field of HF radio data transmission is treated in broad terms. A limited base for use in comparison with HF channel attributes is set up by relating the data-handling characteristics of switched long-distance telephone lines. Generalized criteria of performance for digital data and TTY transmission is reviewed, together with a discussion of what we presently know about the behavior of the HF ionospheric channel with regard to data transmission. A broad and detailed coverage is given to the determination of the required channel characteristics and to the methodology associated with obtaining this information. A description of the USASRDL field test program and some of its results and implications are discussed. Curves of performance of multichannel digitallymodulated FSK and PSK systems are presented. Data-collection and -reduction techniques are described. The over-all program is reviewed in terms of applications of the medium's statistics in a manner which points the way toward future activity in the field of transmission of digital data by HF ionospheric channels. In this regard both channel matching and coding receive their proper recognition and are discussed in terms of providing the promise of significant improvements in the HF radio transmission of digital data.