  SAMP, the Simple Application Messaging Protocol, is a hub-based communication
standard for the exchange of data and control between participating client
applications. It has been developed within the context of the Virtual
Observatory with the aim of enabling specialised data analysis tools to
cooperate as a loosely integrated suite, and is now in use by many and varied
desktop and web-based applications dealing with astronomical data. This paper
reviews the requirements and design principles that led to SAMP's
specification, provides a high-level description of the protocol, and discusses
some of its common and possible future usage patterns, with particular
attention to those factors that have aided its success in practice.
