In an attempt to improve the effectiveness of aid, many of the stakeholders in the international aid regime agreed to commit to five key principles in the Paris Declaration (PD) in 2005. These principles of ownership, alignment, harmonisation, managing for results and mutual accountability were aimed at improving the effective delivery and use of aid, although the Declaration has been followed by continuing doubts over aid effectiveness, especially in the context of deficiencies in donor cooperation and coordination and weak recipient ownership. Since the PD, donors have made varying efforts when it comes to implementing the Paris requirements towards greater aid effectiveness. However, after two OECD DAC monitoring surveys, in 2006 and in 2008, donors and recipients found out that the overall result of the progress of the implementation has been slow and that donor behavioural change towards implementing the PD has differed. In the light of this, this research aims to examine how donors have implemented the PD and why there are such differences in donor behaviour based on a comparative study of Sweden, the United Kingdom (UK), South Korea and China in Tanzania. This thesis reveals that there are key differences between advanced donors (Sweden and the UK) and emerging donors (Korea and China), particularly in terms of their levels of behavioural change in implementing the PD. While Sweden and the UK have shown greater progress in implementing many of the protocols of the PD, Korea and China have barely implemented the Paris requirements. The findings of this research highlight that the uneven responses and outcomes of the PD implementation are due to the design of the PD, which was based on the existing aid delivery mechanism of traditional donors at its top level, and the Paris requirements have not considered the bottom level reality of emerging donors who have different aid mechanisms from traditional donors. By examining seven major factors which inform the uneven donor performance (aid amount and number of staff, aid history of donors, political commitments, action plans and country specific strategies, aid management systems, aid modalities, and monitoring and evaluation), this study argues that the PD has been an 'easy option' for traditional donors such as Sweden and the UK, while it requires radical changes for emerging donors such as Korea and China. While this research relies on the public policy implementation theories to explain uneven donor behaviour in the PD implementation process, there has been less focus on the political economy and the self-interests and motivations of donors, which remains a main limitation of the study. Given this, this research has suggested conducting a further study on donor behaviour with a new methodological focus on the political economy and donor self-interests.