This book was born from the desire to create something meaningful and useful for professionals in the field of process fluid dynamics and industrial plant engineering, as well as for students aiming to specialize in this specific category of products. It is intended for process engineers, instrumentation specialists, designers, and professionals working in related sectors such as pneumatics, hydraulics, actuation systems, pump and compressor manufacturing, or anyone approaching this world for the first time and seeking a deeper understanding of the sizing and selection methods for these pressure-regulating devices, which are virtually always present in industrial plants. Self-operated pressure regulators, despite their seemingly simple design, are often misunderstood or underestimated. They possess a range of important and nuanced characteristics that must be considered for proper understanding and application.