In the information era, the great diversity of application demands calls for the adoption of different wireless communication protocols. As Internet of things (IoT) has gotten dramatic development in recent years, those wireless protocols are included in a common networking framework. With IoT applications getting proliferated, we will witness the co-existence of multiple wireless protocols in the same space, especially in indoor environments. Due to the different communication standards, generally those co-existing protocols cannot directly share information with each other, leading to inevitable interference and degraded network performance. Co-existence of wireless protocols thus becomes a hot topic in both academic and industrial fields. Based on the survey of recent studies in wireless network co-existence, this article illuminates the root causes of the co-existence problem and analyzes its impacts on network designs and performance. The taxonomy of wireless network co-existence is presented, which categories the existing works into three classes: elimination of homogeneous interference, identification of heterogeneous interference, and cross-protocol communication. The potential research directions in this area are further discussed.