This dissertation explores the potential for mass customization in the online fashion industry from both producer and consumer perspectives. It analyzes the advantages of technology, threats from fast fashion, and barriers to consumer adoption of customization. Through a literature review and empirical study including an industry expert interview and consumer survey, the dissertation aims to determine consumers' willingness to customize and buy clothes online and the conditions for a successful customization strategy. Key findings include an assessment of opportunities and risks for startups pursuing customization and a new consumer-focused framework for customization as a business strategy.