As video content on the web continues to expand, it is increasingly important to properly annotate videos for effective search and mining. While the idea of annotating static imagery with keywords is relatively well known, the idea of annotating videos with natural language keywords to enhance search is an important emerging problem with great potential to improve the quality of video search. However, leveraging web-scale video datasets for automated annotation also presents new challenges and requires methods specialized for scalability and efficiency. In this chapter we review specific, state of the art techniques for video analysis, feature extraction and classification suitable for extremely large scale automated video annotation. We also review key algorithms and data structures that make truly large scale video search possible. Drawing from these observations and insights, we present a complete method for automatically augmenting keyword annotations to videos using previous annotations for a large collection of videos. Our approach is designed explicitly to scale to YouTube sized datasets and we present some experiments and analysis for keyword augmentation quality using a corpus of over 1.2 million YouTube videos. We demonstrate how the automated annotation of webscale video collections is indeed feasible, and that an approach combining visual features with existing textual annotations yields better results than unimodal models.