A betting exchange system receiving a plurality of bet requests backing a given outcome, each backing bet request specifying a price and a size at which an originator of the request is prepared to bet that the given outcome will occur. The betting exchange system receiving a plurality of bet requests laying the given outcome, each laying bet request specifying a price and a size at which an originator of the request is prepared to bet that the given outcome will not occur. The betting exchange system aggregating backing bet requests specifying the same price, aggregating laying bet requests specifying the same price, automatically matching a received backing bet request with individual or aggregated laying bet requests to complete a betting transaction, and automatically matching a received laying bet request with individual or aggregated backing bet requests to complete a betting transaction.