This article presents a comparative review on the operability and flexibility concepts and their application to process design and control. First, the operability and flexibility methodologies are summarized. Then the application of the operability framework to steady-state and dynamic systems is illustrated through the examination of several example categories such as linear and nonlinear, square and non-square systems. The flexibility approach based on the active set strategy is used to study the same examples from the flexibility point of view. The discussed results show that the operability and flexibility approaches examine a process from different perspectives and provide valuable complementary information. (C) 2009 American Institute of Chemical Engineers AIChE J, 56: 702-716, 2010