How mature is nonlinear optimization?

Numerical methods for solving nonlinear optimization problems have been developed for over 50 years. Has this field reached maturity? What are the current research frontiers and ongoing challenges? These are questions that this paper attempts to clarify, if not fully answer. The discussion does not explore the technical intricacies of nonlinear optimization techniques, but instead focusses on concepts and research practice. The field's vibrant nature is illustrated by a number of applications, such as adaptive lens design for spectacles, the piloting of injection of dangerous drugs to patients, the identification of parameters in biochemical models of neurons, food sterilisation or animation techniques for video games.