Nanotechnology is enabling the development of devices in a scale ranging from one to a few hundred nanometers. Communication among these nano-devices will expand the capabilities and applications of individual devices both in termsof complexity and range of operation, enabling new applications of nanotechnology in the medical, environmental and military fields as well as in consumer and industrial goods. Despite major progress in the design and manufacturing of these devices has been accomplished to date, it is still not clear how they are going to communicate. Two main alternatives for communication among nano-devices have been envisioned, namely, molecular communication, i.e., the transmission of information encoded in molecules, and nano-electromagnetic communication, which is defined as the transmission and reception of electromagnetic radiation from nanoscale components based on novel nanomaterials. In this paper, propagation models for both communication paradigms are discussed, emphasizing the challenges in nanocommunication networks.