Nano-electromagnetic communication networks, namely, wireless nano networks (WNNs), are wireless communication networks composed of interacting nanonodes (sizes ranging from a few hundred cubic nanometers to micrometers). The extremely limited capabilities and resources of nanonodes, as well as the severe path loss of terahertz band communication in WNNs, represent a challenge to the communication distance among nanonodes and the overall network performance. Therefore, appropriate routing protocols are necessary for guaranteeing multihop communication in WNNs. In this chapter, the existing routing protocols for WNNs are comprehensively analyzed and classified into three categories: limit-flood-areabased routing protocols, dynamic-infrastructure-based (DIF-based) routing protocols, and single-path-based routing protocols. Based on the peculiarities of WNNs, especially the constrained resources and limited energy supply, the features of each protocol are presented through a detailed comparison. Finally, by integrating the features of WNNs and the problems of existing routing technologies, we present our views on the future research directions of routing techniques in WNNs.