Foreign investment is mainly protected through national laws. However the wide-spreading network of bilateral investment treaties aims to ensure a certain standard of protection. These treaties demonstrate far-reaching implications at both treaty level and international level. The implications raise an important question as to whether bilateral investment treaties are coherent or not. Coherence can be viewed as an attempt to prettify the law and minimise the effect of politics which may leave the law incoherent. It is obvious that bilateral investment treaties need to be coherent for a number of reasons. Firstly, incoherent treaties may create problems in relation to the development policy of member countries. Secondly, coherence reassures that negotiators of such treaties would not encounter possible contradictions and inconsistencies amongst the countries' agreement network as well as between the treaties and domestic laws. Thirdly, coherence is critical to treaty interpretation as it is necessary to avoid further complications which may arise from contradictory awards. The aim of this thesis is mainly to elucidate the meaning of coherence and use it to provide an understanding as to how coherent these treaties are. The coherence of bilateral investment treaties will be evaluated in a number of aspects: coherence between bilateral investment treaties and the fundamental principles of international investment law; coherence between bilateral investment treaties and their objectives of investment promotion and investment liberalisation; coherence within the bilateral investment treaties network; coherence between bilateral investment treaties and customary international law on foreign investment; coherence between bilateral investment treaties and free trade agreements; coherence between bilateral investment treaties' obligations and non-investment obligations of states.