We describe a graphical, interactive technique for modeling univariate simulation input processes by using a family of probability distributions based on Bezier curves. This family has an open-ended parameterization and is capable of accurately representing an unlimited variety of distributional shapes. Our input-modeling technique is implemented in a self-contained, Windows-based software system called PRIME (PRobabilistic Input Modeling Environment). Several examples illustrate the advantages of this technique over conventional methods for simulation input modeling.