This book presents comprehensive information on the relay auto-tuning method for unstable systems in process control industries, and introduces a new, refined Ziegler-Nichols method for designing controllers for unstable systems. The relay auto-tuning method is intended to assist graduate students in chemical, electrical, electronics and instrumentation engineering who are engaged in advanced process control. The books main focus is on developing a controller tuning method for scalar and multivariable systems, particularly for unstable processes. It proposes a much simpler technique, avoiding the shortcomings of the popular relay-tuning method. The effects of higher-order harmonics are incorporated, owing to the shape of output waveforms. In turn, the book demonstrates the applicability and effectiveness of the Ziegler-Nichols method through simulations on a number of linear and non-linear unstable systems, confirming that it delivers better performance and robust stability in the presence of uncertainty. The proposed method can also be easily implemented across industries with the help of various auto-tuners available on the market. Offering a professional and modern perspective on profitably and efficiently automating controller tuning, the book will be of interest to graduate students, researchers, and industry professionals alike. M. Chidambaram is currently a Professor at the Department of Chemical Engineering, Indian Institute of Technology in Madras, Chennai. After completing his PhD at the Indian Institute of Science in Bangalore he served as a faculty member at the Indian Institute of Technology Bombay, Mumbai, from 1984 to 1991. Since then he has been a faculty member at the Indian Institute of Technology in Madras. He has also served the institute as Head of the Department of Chemical Engineering from 2000 to 2003 and as the Director, National Institute of Technology (NIT), Tiruchirappalli from 2005 to 2010. He has 190 journal articles, 7 books and 4 book chapters to his credit. His primary research interest is in the area of process control. Nikita Saxena completed her PhD under the guidance of Prof. M Chidambaram. She completed her degree (BTech) in Chemical Technology at Harcourt Butler Technical Institute (HBTI), Kanpur. She also has a year of experience in the fast-moving consumer goods (FMCG) industry. She has authored 4 journal articles and presented papers at several conferences. Her areas of interest include relay control systems and model identification.