Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for classifying objects. Object (e.g., Website) classification can be performed using multiple models that are implemented in a distributed processing environment. In response to receiving a request to classify an object based on attribute values that characterize the object models that are configured to classify objects based on the attribute values are identified and the attribute values are provided to the identified models. Based on classification data received from selected models is used to generate a final classification for the object. The final classification can specify or correspond to a presentation availability status of the object that can be used to control availability of the object. For example, a Website having a final classification of “blacklisted” can be made unavailable for presentation based on the classification result.