Wireless Body Area Networks (WBANs) constitute an emerging technology in the field of health care that makes health monitoring possible from one’s home itself. WBANs open many challenges by placing sensors on/inside human bodies for collecting various health-related information. Unlike traditional Wireless Sensor Networks (WSNs), communication in WBANs suffers from high channel fading and attenuation due to human body fat. Therefore, relay-based communication with data forwarding techniques is used to handle link failures and poor network connectivity. Accordingly, in this survey article, we present a comprehensive study of relay-based communication mechanisms in WBANs. We begin with a brief look at the multi-tiered architecture of WBANs, how direct communication works, and how relay-based communication is different. Subsequently, we present a detailed review of relay node selection approaches, which, in turn, also affects how a WBAN performs. In this context, we also look at the unique quality of service (QoS) demands of WBANs and how they can be assured.