The sugar content of California wine grapes has increased significantly over the past 10–20 years, and this implies a corresponding increase in the alcohol content of wine made with those grapes. In this paper we develop a simple model of winegrape production and quality, including sugar content and other characteristics as choice variables along with yield. Using this model we derive hypotheses about alternative theoretical explanations for the phenomenon of rising sugar content of grapes, including effects of changes in climate and producer responses to changes in consumer demand. We analyze detailed data on changes in the sugar content of California wine grapes at crush to obtain insight into the relative importance of the different influences. We buttress this analysis of sugar content of wine grapes with data on the alcohol content of wine.