Title: DetectBench: An Object Detection Benchmark for OOD Generalization Algorithms. Abstract: The consensus about practical machine learning tasks, such as object detection, is still the test data are drawn from the same distribution as the training data, which is known as IID (Independent and Identically Distributed). However, it can not avoid being confronted with OOD (Out-of-Distribution) scenarios in real practice. It is risky to apply an object detection algorithm without figuring out its OOD generalization performance. On the other hand, a plethora of OOD generalization algorithms has been proposed to amortize the gap between the in-house and open-world performances of machine learning systems. However, their effectiveness was only demonstrated in the image classification tasks. It is still an opening question of how these algorithms perform on complex and practical tasks. In this paper, we first specify the setting of OOD-OD (OOD generalization object detection). Then, we propose DetectBench consisting of four OOD-OD benchmark datasets to evaluate various object detection and OOD generalization algorithms. From extensive experiments on DetectBench, we find that existing OOD generalization algorithms fail dramatically when applied to the more practical object detection tasks. This raises questions over the current progress on a large number of these algorithms and whether they can be effective in practice beyond simple toy examples. For future work, we sincerely hope that DetectBench can serve as a foothold for OOD-OD research.