视觉理解，如物体检测、语义和实例分割以及动作识别等，在人机交互和自动驾驶等领域中有着广泛的应用并发挥着至关重要的作用。近年来，基于全监督学习的深度视觉理解网络取得了显著的性能提升。然而，物体检测、语义和实例分割以及视频动作识别等任务的数据标注往往需要耗费大量的人力和时间成本，已成为限制其广泛应用的一个关键因素。弱监督学习作为一种降低数据标注成本的有效方式，有望对缓解这一问题提供可行的解决方案，因而获得了较多的关注。围绕视觉弱监督学习，本文将以物体检测、语义和实例分割以及动作识别为例综述国内外研究进展，并对其发展方向和应用前景加以讨论分析。在简单回顾通用弱监督学习模型，如多示例学习(multiple instance learning,MIL)和期望—最大化(expectation-maximization,EM)算法的基础上，针对物体检测和定位，从多示例学习、类注意力图机制等方面分别进行总结，并重点回顾了自训练和监督形式转换等方法；针对语义分割任务，根据不同粒度的弱监督形式，如边界框标注、图像级类别标注、线标注或点标注等，对语义分割研究进展进行总结分析，并主要回顾了基于图像级别类别标注和边界框标注的弱监督实例分割方法；针对视频动作识别，从电影脚本、动作序列、视频级类别标签和单帧标签等弱监督形式，对弱监督视频动作识别的模型与算法进行回顾，并讨论了各种弱监督形式在实际应用中的可行性。在此基础上，进一步讨论视觉弱监督学习面临的挑战和发展趋势，旨在为相关研究提供参考。

Visual understanding, e.g., object detection, semantic/instance segmentation, and action recognition, plays a crucial role in many real-world applications including human-machine interaction, autonomous driving, etc. Recently, deep networks have made great progress in these tasks under the full supervision regime. Based on convolutional neural network (CNN), a series of representative deep models have been developed for these visual understanding tasks, e.g., you only look once (YOLO) and Fast/Faster R-CNN(region CNN) for object detection, fully convolutional networks(FCN) and DeepLab for semantic segmentation, Mask R-CNN and you only look at coefficients (YOLACT) for instance segmentation. Recently, driven by novel network backbone, e.g., Transformer, the performance of these tasks have been further boosted under full supervision regime. However, supervised learning relies on massive accurate annotations, which are usually laborious and costly. By taking semantic segmentation as an example, it is very laborious and costly for collecting dense annotations, i.e., pixel-wise segmentation masks, while weak supervision annotations, e.g., bounding box annotations, point annotations, are much easier to collect. Moreover, for video action recognition, the scenes in videos are very complicated, and it is very likely to be impossible to annotate all the actions with accurate time intervals. Alternatively, weakly supervised learning is effective in reducing the cost of data annotations, and thus is very important to the development and applications of visual understanding. Taking object detection, semantic/instance segmentation, and action recognition as examples, this article aims to provide a survey on recent progress in weakly supervised visual understanding, while pointing out several challenges and opportunities. To begin with, we first introduce two representative weakly supervised learning methods, including multiple instance learning (MIL) and expectation-maximization (EM) algorithms. Despite of different network architectures in recent weakly supervised learning methods, most existing methods can be categorized into the family of MIL or EM. As for object localization and detection, we respectively review the methods based on MIL and class attention map (CAM), where self-training and switching between supervision settings are specifically introduced. By formulating weakly supervised object detection (WSOD) as the problem of MIL-based proposal classification, WSOD methods tend to focus on discriminative parts of object, e.g., head for human or animals may be simply detected to represent the entire object, yielding significant performance drops in comparison to fully supervised object detection. To address this issue, self-training and switching between supervision settings have been respectively developed, and transfer learning has also been introduced to exploit auxiliary information from other tasks, e.g., semantic segmentation. As for weakly supervised object localization, CAM is a popular solution to predict the object position where objects from one class with the highest activation value can be found. Similarly, CAM based localization methods are also facing the issue of discriminative parts, and several solutions, e.g., suppressing the most discriminative parts and attention-based self-produced guidance, have been proposed. Based on pattern analysis, statistical modeling and computational learning visual object classes(PASCAL VOC) and Microsoft common objects in context(MS COCO) datasets, several representative weakly supervised object localization and detection methods have been evaluated, showing performance gaps between fully supervised methods. As for semantic segmentation, we consider several representative weak supervision settings including bounding box annotations, image-level class annotations, point or scribble annotations. In comparison to segmentation mask annotations, these weak annotations cannot provide accurate pixel-wise supervision. Image-level class annotations are the most convenient and easiest way, and the key issue of image-level weakly supervised semantic segmentation methods is to exploit the correlation between class labels and segmentation masks. Based on CAM, coarse segmentation results can be obtained, while facing inaccurate segmentation masks and focusing on discriminative parts. To refine segmentation masks, several strategies are introduced including iterative erasing, learning similarity between pixels, and joint learning of saliency detection and weakly supervised semantic segmentation. Point or scribble annotations and bounding box annotations can provide more accurate localization information than image-level class annotations. Among them, bounding box annotations is likely to be a good solution to balance the annotation cost and performance of weakly supervised semantic segmentation under EM framework. Moreover, weakly supervised instance segmentation is more challenging than weakly supervised semantic segmentation, since a pixel is not only assigned to an object class but also is accurately assigned to one specific object. In this article, we consider bounding box annotations and image-level annotations for weakly supervised instance segmentation. Based on image-level class annotations, peak response map in CAM is highly correlated with object instances, and can be adopted in weakly supervised instance segmentation. Based on bounding box annotations, weakly supervised instance segmentation can be formulated as MIL, where instance masks are usually more accurate than those based on image-level class annotations. Besides, in these weakly supervised segmentation methods, post-processing techniques, e.g., dense conditional random field, are usually adopted to further refine the segmentation masks. On PASCAL VOC and MS COCO datasets, representative weakly supervised semantic and instance segmentation methods with different levels of annotations are evaluated. As for video action recognition, it is much more difficult to collect accurate annotations of all the actions due to complicated scenes in videos, and thus weakly supervised action recognition is attracting research attention in recent years. In this article, we introduce the models and algorithms for different weak supervision settings including film scripts, action sequences, video-level class labels and single-frame labels. Finally, the challenges and opportunities are analyzed and discussed. For these visual understanding tasks, the performance of weakly supervised methods still has improvement room in comparison to fully supervised methods. When applying in the wild, it is also a valuable and challenging topic to exploit large amount of unlabeled and noisy data. In future, weakly supervised visual understanding methods also will benefit from multi-task learning and large-scale pre-trained models. For an example, vision and language pre-trained models, e.g., contrastive language-image pre-training (CLIP), is potential to provide knowledge to significantly improve the performance of weakly supervised visual understanding tasks.