This article describes middleware by adopting a horizontal and a vertical layer views. Middleware are enabling technologies for application development and execution in ubiquitous environments. In the horizontal view, we find most types of middleware developed so far, such as MOM, ORB, databases middleware and more recently SOA. Two new concepts emerged in this category, the ''middleware of sensors" and the ''middleware of middlewares". The vertical layer distinguishes two essential properties that characterize nowadays middleware and define what is called to be the ubiquitous middleware and the self-* middleware. This article briefly explains all these key words and exposes the directions in which these new areas of research could proceed. It than introduces the spontaneous middleware as a new property for ubiquity in the vertical layer and highlights what spontaneity brings to the different types of middleware.