Clouds are becoming the preferred platforms for large-scale applications. Currently, Cloud environments focus on high scalability and availability by relaxing consistency. Weak consistency's considered to be sufficient for most of the currently deployed applications in the Cloud. However, the Cloud is increasingly being promoted as environment for running a wide range of different types of applications on top of replicated data - of which not all will be satisfied with weak consistency. Strong consistency, even though demanded by applications, decreases availability and is costly to enforce from both a performance and monetary point of view. On the other hand, weak consistency may generate high costs due to the access to inconsistent data. In this paper, we present a novel approach, called cost-based concurrency control (C3), that allows to dynamically and adaptively switch at runtime between different consistency levels of transactions. C3 has been implemented in a Data-as-a-Service Cloud environment and considers all costs that incur during execution. These costs are determined by infrastructure costs for running a transaction in a certain consistency level (called consistency costs) and, optionally, by additional application-specific costs for compensating the effects of accessing inconsistent data (called inconsistency costs).C3 considers transaction mixes running different consistency levels at the same time while enforcing the inherent consistency guarantees of each of these protocols. The main contribution of this paper is threefold. First, it thoroughly analyzes the consistency costs of the most common concurrency control protocols; second, it specifies a set of rules that allow to dynamically select the most appropriate consistency level with the goal of minimizing the overall costs (consistency and inconsistency costs);third, it provides a protocol that guarantees that anomalies in the transaction mixes supported by C3 are avoided and that enforces the c- rrect execution of all transactions in a transaction mix. We have evaluated C3 on the basis of real infrastructure costs, derived from Amazon's EC2. The results demonstrate the feasibility of the cost model and show that C3 leads to a reduction of the overall costs of transactions compared to a fixed consistency level.