This paper assesses the comparative study of the role of tax audit and tax investigations in developed and developing countries (emphasis on Nigeria and United Kingdom). The two research questions that guided the research include; to what extent does effective tax audit and investigation curb incidents of tax evasion in developed countries?; to what extent does effective tax audit and investigation in developed countries enhance their tax revenue generation?. For developing countries, Nigeria as a case study, the methodology employed for data collection is only primary source, which involved the use of questionnaires, in which 48 questionnaire were administered to the members of staff Port Harcourt branch of Federal Inland Revenue Services (FIRS) and all the questionnaire were returned. Conversely, for developed countries, a critical look was given to published journals and some published information on databases that have confirmed that tax evasion occurs less often in UK owing to effective tax audit and investigation. Such journals have equally confirmed that one of the reasons for increase in revenue derivable from tax is as a result of effective tax audit and investigation as well as reliable tax laws. A Simple random sampling technique was adopted in distributing 48 copies of questionnaire, which were returned. Presentation and analysis of primary and secondary data collected with questionnaire were done using table, charts and simple percentages. It was established that effective and efficient tax and investigation has not been optimally put in place in developing countries hence the rampant incidents of tax evasion and low revenue generated through tax administration systems in such countries. On the other hand, in developed countries such as UK, tax audit and investigation is effective and efficient and for that reason, higher revenue is generated and less incidents of tax evasion. The study therefore concludes that for increased revenue generation and prevention of tax evasion, reliable, efficient and effective tax audit and investigation should be put in place in both developing and developed economies. It is the recommendation of the study that developing countries urgently embrace functional audit and investigation exercises to increase their revenue base through taxation and prevent uncontrollable loss of revenue through tax evasion while developed countries should maintain and strengthen their tax audit and investigation activities for a sustained optimal performance.