This paper is derived from a study that was funded by Kenyatta university Vice-Chancellor's Research Grant. It discusses the activities of organized criminal groups as well as the effects of those activities on security and peace in Nairobi. Nairobi County was chosen for the study because it has the highest number of organized criminal groups in the country. Such groups pose a serious challenge to Kenya Government for it negates the main objective of vision 2030 which is to achieve a secure environment for sustainable social, economic, and political development. The study used oral interviews, participant observation, and document analysis as methods of data collection. Findings of the study revealed that criminal groups were involved in several activities which made the streets, places of work or residence, public transport, and open-air markets unsafe. It was concluded that the Kenya Government must invest more in security so that Nairobi residents can live peacefully and carry out their businesses without fear of being attacked by criminals. From the findings and conclusions of the study, recommendations were made on how the problem of organized criminal groups in Nairobi can be minimized to ensure residents can live in a peaceful and secure county.