Access to modern forms of energy has been recognized as a prerequisite to achieve sustainable development and overall improvement in the quality of life. In the literature, there exist very few studies that are focused on access to and use of modern forms of energy by the urban poor in developing countries. The aim of this study is to examine the energy use pattern of slum-dwellers in two cities of Thailand, Bangkok and Khon Kaen, the energy expenditures, and the key factors for the high access to electricity and other modern forms of energy. This study finds that almost 100% of the households in the slums of Bangkok and Khon Kaen have electricity connections. Also, a very high percentage of slum-dwellers (about 87% in Bangkok and 86% in Khon Kaen) use LPG for cooking. Slum-dwellers spend about 16% of their monthly income on energy in Bangkok and about 26% in Khon Kaen. Cooking energy is found to represent a relatively small share in the total household income of the slum households. The factors behind the high access of the urban poor to modern forms of energy in Thailand were the successful electrification program, price subsidies and the low monthly electricity service charge for the poor households, and reliable supply and ready availability of LPG.