Path loss models (PLMs) play a fundamental role in the deployment of wireless sensor networks (WSNs) since they support tasks related to positioning, tracking, and power control, making the end nodes coexist in complex scenarios. Such models are used on design to choose the transmission parameters, guaranteeing that the waves can be demodulated at the receiver. Different models have been proposed to quantify the path loss for general applications; however, WSNs exhibit constraints that make its application unsuitable regarding prediction accuracy. These constraints motivate proposing niche-specific PLMs that meet the WSNs' requirements. This paper identifies the trends in PLMs for WSNs. We have found the variables considered in the models, typical scenarios, reference models used to assess each approach, the most used frequency bands, specific modulations, and the research opportunities and open problems. This characterisation helps designers to establish the PLMs to achieve energy savings, accurate positioning, and reliable links.