Performance is one of the fundamental factors in the design, development and configuration of computer and communication systems. Performance evaluation therefore has been and continues to be of great practical importance in the computer industry. Of the two main approaches to performance evaluation, namely measurement and modeling, modeling has received the greatest research attention, both in industry and in universities. Hence, performance modeling is the subject of this paper. The paper is not intended to be a survey of performance modeling in industry. Rather we present our personal perspectives based on the development and application of performance modeling in IBM over the past 30 years.