Manufacturing Execution Systems (MES) have evolved from manufacturing control systems. The driving force is the increased requirements for mass customization  and a higher number of customer options. As the name implies, MES have an emphasis on Execution not just passive data collection. This paper looks at the functionality and architecture of MES applications. The primary conclusion is that MES are no more a silver bullet than other software fads have been over the last 30 years. The benefits come to those organizations willing to act on the information and go beyond just installing another software package.