GeSn has emerged as a promising semiconductor with optoelectronic functionality in the mid-infrared, with the potential of replacing expensive III-V technology for monolithic on-chip Si photonics. Multiple challenges to achieve optoelectronic-grade GeSn have been successfully solved in the last decade. We stand today on the brink of a potential revolution in which GeSn could be used in many optoelectronic applications such as light detection and ranging devices and lasers. However, the limited understanding and control of material defects represents today a bottleneck in the performance of GeSn-based devices, hindering their commercialization. Point and linear defects in GeSn have a strong impact on its electronic properties, namely, unintentional doping concentration, carrier lifetime, and mobility, which ultimately determine the performance of optoelectronic devices. In this review, after introducing the state-of-the-art of the fabrication and properties of GeSn, we provide a comprehensive overview of the current understanding of GeSn defects and their influence on the material (opto)electronic properties. Where relevant, we also review the work realized on pure Ge. Throughout the manuscript, we highlight the critical points that are still to solve. By bringing together the different fabrication techniques available and characterizations realized, we offer a wholistic view on the field of GeSn and provide elements on how it could move forward.