Model-based small area estimation is frequently used in conjunction with survey data in order to establish estimates for under-sampled or unsampled geographies. These models can be specified at either the area-level, or the unit-level, but unit-level models often offer potential advantages such as more precise estimates and easy spatial aggregation. Nevertheless, relative to area-level models literature on unit-level models is less prevalent. In modeling small areas at the unit level, challenges often arise as a consequence of the informative sampling mechanism used to collect the survey data. This paper provides a comprehensive methodological review for unit-level models under informative sampling, with an emphasis on Bayesian approaches. To provide insight into the differences between methods, we conduct a simulation study that compares several of the described approaches. In addition, the methods used for simulation are further illustrated through an application to the American Community Survey. Finally, we present several extensions and areas for future research. Unit Level Modeling of Survey Data for Small Area Estimation Under Informative Sampling: A Comprehensive Overview with Extensions