The main problem hampering innovation in the Internet of Things (IoT) is the fragmentation and lack of interoperability between IoT platforms. A possible solution for IoT platform sponsors to overcome this problem is to open up towards other platforms. To better understand how the IoT platform market is evolving and to inform future decisions regarding the desired degree of openness between IoT platforms, this thesis aimed to develop a theory on the openness between IoT platforms by identifying, prioritizing and theorizing the interrelations between factors driving the decisions from IoT platform owners related to the openness of their platform towards other IoT platforms. To this end, a preliminary theoretical framework was developed which was used as input for 13 semi-structured interviews with decision makers and field experts. It was found that openness between platforms is mostly driven by complementarities. Due to the cyber-physical nature of the IoT, the domain is characterised by a high need for specialisation and platforms are often developed from a product-centric, bottom-up approach. This results in a fragmented market in which there are strong complementarities between IoT platforms. It is found that these complementarities are the main factor driving the openness between IoT platforms.