A Supply-Side Platform (SSP) is a platform through which publishers make available and sell their ad space in the programmatic model. On the other side of the auction stand advertisers with a DSP, and the SSP makes sure the publisher sells the space as favourably as possible. It is the mirror image of the DSP: one side buys, the other sells, and they meet at the exchange (Ad Exchange). (Compare: DSP, Ad Exchange, RTB)