Thailand’s foreign policy is reputed to be accommodating and pragmatic towards external environments. This character renders a name “bamboo bend-ing with the wind” diplomacy which portrays the position as “always solidly rooted, but flexible enough to bend whichever way the wind blows in order to survive” (Kislenko, 2002, p. 537). This image corresponds to a nature of small states in International Relations where state’s survival in an anarchical world is an ultimate goal. However, such a view has become problematic in explaining Thai foreign policy in the post-Cold War era. This chapter therefore contends that Thai foreign policy behavior has become more proactive so as to assert its leadership in Southeast Asia. The main concern of this chapter is to show how Thailand has attempted to achieve its regional leadership goal through major foreign policy actions in the post-Cold War era. However, Thailand’s regional leadership oscillates mainly due to its domestic factors