This paper presents an overview of the horizontal approach for the creation of Internet of Things (IoT) platforms, which allows to overcome many of the disadvantages of the vertical approach, by allowing a provider to supply only a horizontal slice in the delivery of IoT services, thus meeting better today’s expectations for efficiency and effectiveness, flexibility and scalability, openness and easy adjustment to new use cases and application scenarios, and multi-purpose use of the IoT systems built on top of such platforms. The state of the art in the area of IoT platforms development is presented along with some examples of horizontal IoT platform solutions in use today. The EMULSION IoT platform, developed by following the horizontal approach, is briefly described at the end.