The design of efficient controllers for nonlinear plants, which can be problematic because of challenges related to real world applications, has received a tremendous attention from researchers since many years. However, there is still a need for addressing some issues that remain not clearly solved related to limitations imposed by some assumptions or some complicated approaches hard to apply for practitioners. There is also a need for approaches able to handle simultaneously several potential issues without any unnecessary additional energy usage and with improved control performances. Therefore, this book focuses on addressing the design of efficient adaptive nonlinear controllers for strict-feedback nonlinear systems by taking into account multiple challenges for increased safety and reliability. The considered challenges are external disturbances, uncertain dynamics, actuation faults, unmeasured states, constrained input, unknown control direction, and singularity in the control law.