Thermoelectric generators are devices that directly convert heat flux or the temperature difference between two hot and cold surfaces into electricity. With the advancement of the Internet of Things (IoT) technology and the development of wearable and portable devices, the issue of providing a sustainable power source is one of the main challenges in the development path of these tools. Creating electric power by harvesting the waste heat from the human body is one of the effective solutions in this way. For this reason, the development and improvement of the technology of wearable thermoelectric generators have received much attention recently. Due to the low-temperature difference between the two sides of wearable thermoelectric generators and the high thermal resistance between the skin and the heated surface of these modules, the performance of these systems is highly dependent on their structural parameters and environmental factors. In this paper, it has tried to review all the previous studies regarding the impact of structural factors (such as the matching of internal and external thermal resistances, geometrical parameters of the module, design of heat source and sink, and flexibility of thermoelectric module) and environmental parameters (including the effect of ambient air temperature and humidity, skin temperature, and the interaction of power consumers with thermoelectric modules). Based on the studies, it seems that in optimizing the performance of wearable thermoelectric generators (WTEGs), it is necessary to consider the effect of the human body's thermoregulatory responses, such as skin temperature and sweating rate. The change in skin temperature directly affects the performance of WTEGs, and the change in sweating rate can also affect the thermal resistance between the skin and the hot plate and overshadow the matching of thermal resistances during operation.