The product line approach provides a systematic approach for software reuse. A challenge with modeling adaptive and evolvable software product lines is how to model variability. This paper describes four different approaches to modeling variability, modeling variability using parameterization, modeling variability using information hiding, modeling variability using inheritance, and modeling variability using variation points. The variation point model (VPM) is used to demonstrate the fourth approach. VPM allows a reuser or application engineer to extend components at pre-specified variation points. For this to be possible, a variation point must be modeled such that the reuser has enough knowledge to build a variant.