Cloud storage service is currently becoming a very popular solution for medium-sized and startup companies. However, there are still few suitable solutions being offered to deploy transactional databases in a cloud platform. The maintenance of ACID (Atomicity, Consistency, Isolation and Durability) properties is the primary obstacle to the implementation of transactional cloud databases. The main features of cloud computing: scalability, availability and reliability are achieved by sacrificing consistency. The cost of consistency is one of the key issues in cloud transactional databases that must be addressed. While different forms of consistent states have been introduced, they do not address the needs of many database applications. In this dissertation we propose a tree-based consistency approach, called TBC, that reduces interdependency among replica servers to minimize the response time of cloud databases and to maximize the performance of those applications. We compare different techniques of maintaining consistency, including the classic approach, the quorum approach and our tree-based consistency approach. We identify the key controlling parameters of consistency maintenance in cloud databases and study the behavior of the different techniques with respect to those parameters. Experimental results indicate that our TBC approach reduces interdependency between data replicas and has good performance. We also implement a transaction management system using TBC as the consistency approach. We have designed a hierarchical lock manager that is able to work at a variable granularity level and allow much more concurrent access to the data items than regular lock managers. The TBC transaction management system ensures serializability and guarantees the ACID properties. The common isolation problems in transaction management are prevented, and we prove that the scenarios of dirty read, unrepeatable read and dirty write or lost update will never occur in concurrent execution of the transactions. We also present an efficient auto-scaling feature for the proposed transaction manager. Our experimental results shows that TBC has better response time than other approaches regardless of the arrival rate, read-write ratio, variation in data selection preference or database size. The Tree-Based Consistency approach is a viable solution for ACID transactional database management in a cloud.