This paper discusses the issue of uncertainty that occurs in the Internet of Things (IoT) environment. The main operation of IoT includes data transmission over sensors and networks via the Internet. There are three types of heterogeneity; namely heterogeneity of data, devices, and networks, which may be the causes that trigger uncertainty in this environment. Based on the study, there are three uncertainty levels that can occur, which are uncertain schema mapping, uncertain data, and uncertain query. As an example, heterogeneous devices which are not compatible with IoT applications will create an opportunity for uncertain data where the data transmitted from the sensor to the application may be partially missing. System failure is likely to happen when these sources of uncertainty generate incorrect inferences and conclusions, thus producing unreliable information. This paper reviews previous researches to find the most efficient approach to handle uncertainty. Discussion is centred on the efficiencies and drawbacks of different uncertainty handling approaches in different domains of knowledge related to IoT while the most relevant of the works are addressed in detail.