Iterative control structures (loops) allow repeated execution of tasks, activities or subprocess according the given conditions in a process model. However, due to the complexity in managing repeated execution, not all types of loops are fully supported in workflow management systems (WfMS). Most of the workflow engines adhere to structured process models and do not allow some of the iterative control structures such as arbitrary loops. However, a workflow designer may wish to include these loops in their models to better capture the actual business logic. In these situations, these models cannot be executed in majority of WfMS. In this thesis, we propose novel approaches to transform iterative control structures into behaviorally equivalent forms so that they can be executed in WfMS which do not support loops. The proposed methods can be used to unfold structured loops, nested-loops and crossing-loops. The step-by-step unfolding process of the proposed methods is demonstrated in detail using various process models containing different types of loops. Our method is based on workflow graphs and therefore it is compatible with the modeling languages such as Business Process Modelling Notation (BPMN).