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Attention is drawn to the importance of the sampling frequency as a design parameter of sampled-data feedback control systems and consideration is given to its implication to the ability of the systems to withstand gross variations of the plant without unacceptable deviation from the desired performance. It is seen that the robustness of a system is intimately related to the sampling frequency as a consequence of the limitations which it imposes on the achievable loop bandwidth. Conversely, for any specified degree of robustness, the minimum frequency of sampling is well defined. However, for plants with high-frequency dynamics such as the electro-hydraulic servo discussed within the paper, time constraints imposed by a dedicated microprocessor add to the restrictions of the sampling process in such a way that it may not be possible to guarantee the existence of a discrete compensator to satisfy the given specifications.