There are two main fields of application of pulse-modulated systems, communications and control. Communication is not a subject of our concern in this book. Controlling by a pulse-modulated feed­ attracted our efforts. The peculiarity of this book is that all back the sampled-data systems are considered in continuous time, so no discrete time schemes are presented. And finally, we pay a little at­ tention to pulse-amplitude modulation which was treated in a vast number of publications. The primary fields of our interest are pulse­ width, pulse-frequency, and pulse-phase modulated control systems. The study of such systems meets with substantial difficulties. An engineer, who embarks on theoretical investigations of a pulse-mo­ dulated control, is often embarrassed by the sophisticated mathe­ matical tools he needs to know. When a mathematician, who looks for practical applications of his mathematical machinery, meets with these systems, he faces a lot of of complicated technical schemes and terms. Probably this is the reason why publications on pulse modu­ lation are seldom in scientific journals. As for books on this subject (save on amplitude modulation), the significant part of them is in Russian and hardly available for a non-Russian reader.