This chapter examines the transition of Thailand to energy and climate security as an energy consuming country. It explains the links between the historical and contemporary concepts of energy security, modernity and sustainability in Thailand and the broader region. The chapter considers the risks to Thailand of a warming climate and the effects on monsoon variability. It explores activism over Thailand's fossil fuel energy projects focusing on two case studies: The unsuccessful campaign against the Trans-Thai Malaysian Gas Pipeline; and the successful campaign against the Bo Nok coal-fired power plant. The history of energy and energy security is related to ideas about modernity. The relationship between modernity and energy security is not one-directional; rather, they are mutually constitutive. In Southeast Asia, the developments of fossil fuel and electricity systems have often been strongly related to dependency, colonialism and state-building, aspects which often hang together. The expansion of energy systems has always been a matter of energy security in Southeast Asia.