In-network aggregation mechanisms for vehicular ad hoc networks (VANETs) aim at improving communication efficiency by summarizing information that is exchanged between vehicles. Summaries are calculated, while data items are generated in and forwarded through the network. Due to its high bandwidth saving potential, aggregation is a vital building block for many of the applications envisioned in VANETs. At the same time, the specific environment of VANETs calls for novel approaches to aggregation, which address their challenging requirements. In this paper, we survey and structure this active research field. We propose a generic model to describe and classify the proposed approaches, and we identify future research challenges.