Media substitution system

A media substitution system comprising a decisioning service in communication with a plurality of clients, each said client being arranged to receive at least one media channel having a break defined therein, the break comprising at least one substitutable spot containing a linear media item, and, for the or each substitutable spot, send a decisioning request to the decisioning service, the decisioning request including client data specific to that client;

• wherein the decisioning service is arranged to:
• in an initial processing step, determine from the decisioning request whether the linear media item should be output in the corresponding substitutable spot, and if so, provide a decisioning response to the client indicating that no substitution should be performed in the corresponding substitutable spot;
• and otherwise, in a further processing step, determine a media item to be output in the corresponding substitutable spot, and provide a decisioning response to the client indicating the determined item, wherein the further processing step is based on the decisioning request and on additional data not contained in the decisioning request;
• wherein the client is arranged to receive the decisioning response and to output the media item or items in the or each substitutable spot, as indicated by the decisioning response.