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A brief introduction is given to the functions of detection and classification of underwater objects by sonars which scan large volumes of water. For classification purposes the system must have high angular resolution in at least one plane, and this, coupled with high-speed scanning to cover the width of a sector within the time duration of the transmit pulse, typifies what is called `within-pulse scanning¿¿. A brief description of the general theory of high-speed scanning by electronic means leads to practical methods for their implementation followed by some details of systems developed and used for various purposes. In addition to the scanning receivers, this includes other parts essential to the systems, such as transmitters, transducers, stabilisation and display. Some limitations of the water medium are mentioned before the various applications of within-pulse sector-scanning sonar to fisheries research are described.