The 1909-1918 era can be regarded as the formative years of MI5, as it developed from a small counter-espionage bureau into an established security intelligence agency. MI5 had two main roles during this period; counter-espionage, and advising the War Office on how to deal with the police and the civilian population, particularly aliens. Most of the existing literature tends to focus on the development of MI5 as a whole and pays little attention to the six individual branches that constituted MI5 by the armistice. Recently released MI5 documents in The National Archives (rnA) make it possible to examine MI5 at the micro level and set out the intimate workings of its six branches. The study examines the evolution of MI5 from its formation in October 1909 to the end of the First World War in November 1918, paying particular attention to three questions. First, what did a map of the structure of the MI5 organisation look like and "how" did it develop during these years? Secondly, "why" did it develop as it did? Thirdly, "how effective" was MI5 throughout this period? MI5 began as a one-man affair in 1909, tasked with the limited remit of ascertaining the extent of Gemlan espionage in Britain and an uncertain future. By the armistice MI5's role had expanded considerably and it had begun to develop into an established security intelligence agency, with 844 personnel spread over six branches covering the investigation of espionage, prevention, records, ports and travellers, overseas, and alien workers. This study suggests that the main driver of these developments, if one key factor can be singled out, was the changing perception of the nature of the threat posed by German espionage. However, because some within official circles equated all forms of opposition to Government policy with support for Germany, increasing attention also began to be paid to the possibility that industrial umest, pacifists and others who opposed the Government might actually be being directed by a German "hidden hand". From 1917 onwards MI5's development was driven by a conviction that it had defeated German espionage, such that Germany had switched its efforts to promoting Bolshevism and other forms of umest in order to undermine British society. However, MI5's activities were restricted to investigating if there was really any enemy influence behind such things, while Special Branch was to focus on labour unrest generally. This study makes an original and useful contribution to knowledge in three noteworthy respects. First, it sets out probably the most detailed description of MI5's organisational structure available. Secondly, it poses the stimulating question of "how to measure" the effectiveness of a counter-espionage agency? Thirdly, it suggests that, contrary to claims that Vemon Kell was an "empire builder" who wanted a greater role in labour intelligence, Kell felt it appropriate that MI5's activities should be restricted to the investigation of cases of peace propaganda and sedition that arose from enemy activities and did not actually want MI5 to assume a broader role in labour intelligence at that time.