Though the benefits of Six Sigma were widely reported in many large organizations, research had shown its implementation in the UK small and medium-sized enterprises (SMEs) was still less evident. The aim of this exploratory research was to assess the status of Six Sigma implementation in the UK manufacturing SMEs and thereby develop a customized practical framework to facilitate successful implementation of Six Sigma in SMEs. A mixed method approach of survey and multiple case studies during three phases of research was adopted to achieve the aim of the research by answering the following key research questions (RQs): RQ1: What makes SMEs different from large organizations?; RQ2: What are the critical differences in quality management practices of Six Sigma and non-Six Sigma SMEs?; RQ3: What are the critical success factors and barriers to implementation of Six Sigma in SMEs?; RQ4: Does the performance of Six Sigma firms differ from non-Six Sigma firms?; RQ5: How to assess the readiness of a SME to embark on Six Sigma journey?. The adoption of a mixed method approach not only facilitated in answering the five RQs and but also addressed the quality research criteria of reliability and validity for this research. This exploratory research had made some significant contributions to the theory and practice of Six Sigma research in SMEs. This is among very few studies in quality management (QM) literature that presents the differences in the characteristics of SMEs and large organizations through the lens of small business growth models. It also identified the similarities in the critical success factors (CSFs) stated for small business growth and for the implementation of continuous improvement (CI) initiatives in SMEs. The research dispels the myth that Six Sigma implementation is limited to large organizations only. The empirical research had indicated successful implementation of Six Sigma in SMEs of sizes ranging from 35 to 240 headcount. A well-designed quality management system (QMS) based on the principles of ISO 9000 could be the foundation to embark on Six Sigma journey. Networking with government bodies or academic institutions and the role of middle managers were identified as two new factors for successful implementation of Six Sigma in SMEs. This was among very few studies that compared and identified significant differences in the performance of Six Sigma SMEs compared to non-Six Sigma SMEs. The Six Sigma firms out-classed non-Six Sigma firms with respect to the nine performance metrics established from the literature. The two key practical contributions of this doctoral research were the construction of a Six Sigma Readiness Index (SSRI) and a customized Six Sigma framework for SMEs based on the findings from empirical research and literature. The SSRI can assess SME preparedness for Six Sigma implementation. The proposed framework would help SMEs to get started with Six Sigma implementation. The readiness index and framework were tested in three SMEs to assess its robustness and validity. The generalisability of the findings was limited due to the smaller sample size of participating firms from the UK manufacturing sector only. Future research will expand the scope of the study by focusing on global SMEs (manufacturing and services) and conducting exploratory and explanatory research on Six Sigma implementation in SMEs. The author will also test the proposed SSRI and the framework for further refinement and validity.