This paper is the introduction for a special issue which examines intra-party factions and factionalism in competitive party systems of Southeast Asia, looking at the cases of Cambodia, Indonesia, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Timor-Leste, in that order and rounding up with a comparative conclusion. The study centres primarily upon one query: in competitive party systems of Southeast Asia, what accounts for the rise of factionalism in some party systems relative to others? The paper at hand frames this special issue, reviewing the literature and examining the causes, effects and forms of factionalism in general and more specifically in Southeast Asia.