Collaborative editing systems (or group editors) allow a geographically dispersed group of human users to view and modify shared multimedia documents, such as research papers, design diagrams, web pages and source code together over a computer network. In addition to being useful tools, group editors are a classic research vehicle and model of interactive groupware applications, based on which a variety of social and technical issues have been investigated. Consistency maintenance as a fundamental problem in group editors has attracted constant research attention. Operational transformation (OT) is an optimistic consistency maintenance method that supports unconstrained collaboration among human users. Although significant progress has been achieved over the past decade, there is still a large space for improvement on the theoretical part of OT. In this dissertation, we are concerned with three problems: (1) How to evaluate the correctness of OT-based consistency maintenance protocols; (2) How to design and prove correct OT-based protocols; (3) What are the consistency correctness conditions for group editing systems in general. This dissertation addresses the above three problems and makes the following contributions: (1) propose a total order based framework including a new consistency model and the associated design methodology. This framework reduces the complexities of the OT design; (2) improve the total order based framework by introducing a natural order based framework. In contrast, this framework removes the requirement of defining a total order that is not necessary to the OT design; (3) establish a generic consistency model and propose the first set of practical design guidelines in OT based on this model.