The ubiquity of information and communication technologies (ICT) stimulate collaborative shopping and bring together customers with similar interests around the world to perform cooperative and collective online shopping. As a result, various models of collaborative online group buying are emerging. This paper analyses the phases of interaction during the shopping processes and provides a systematic categorization of the online group buying based on the topdown/ bottom-up crowd-shopping models. In addition, this paper discusses recent developments and challenges of group buying in order to contribute to future research directions within the field of collaborative online group buying.