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Predicting phenological stages through modeling has significant implications for planning viticultural practices and for predicting the impact of climate change on phenology. The Chill Overlap Model is based on an exponentially declining curve that integrates the demonstrated compensatory relationship between chill and heat accumulation. It also incorporates recent research-based knowledge of physiological changes during dormancy. The aim of this work was to develop parameters for a Chill Overlap Model to predict budbreak in Vitis vinifera cv. Chardonnay grapevines. We also wanted to determine if using a Chill Overlap Model could be better at predicting budbreak than previously developed phenology models. The Chill Overlap Model incorporated the use of the Dynamic chill accumulation model to quantify accumulation of chill exposure in a cultivar with a relatively low chill requirement. Budbreak timing determined in Californian and Spanish winegrape-growing regions, which have a wide range of climates, was used to establish parameters for a Chill Overlap Model for Chardonnay. The newly developed Chardonnay Chill Overlap Model did not predict budbreak better than previous models, but did highlight significant differences between the dynamics of chilling in grapevines and that in other species for which a Chill Overlap Model has been developed. Further research is needed to understand the environmental and vineyard management factors that influence the timing of budbreak to improve the model and better understand factors that influence the completion of dormancy in grapevines.