Researchers in consumer complaint behaviour agree that consumers’ complaint intentions are influenced by online service failures. These online service failures include delivery problems, payment security problems, web-page navigational problems, product information problems, and customer service problems. Online service failures cause customer dissatisfaction that could trigger consumer complaint behaviour. Most consumer complaint behaviour researchers concur that the purchasing environment primarily influences consumers’ complaint behavioural intentions. Relevant literature on Millennial consumers and theory on service failures and consumer complaint behaviour was integrated to provide this study's theoretical grounding. This study aimed to determine the Millennial consumers' complaint behavioural intentions following a service failure in the online clothing retail context. This study also determined differences in Emerging, Young and Older Millennials complaint behavioural intentions. A survey research design was implemented, using a structured, self-administered online questionnaire to measure Millennial consumers’ (n = 193) complaint behavioural intentions following a service failure in the online clothing retail context. The self-administered questionnaire made use of existing scales and self-developed questions. Millennial consumers indicated that delivery problems would cause them the most dissatisfaction, followed by payment security problems, customer service problems, product information problems, and webpage navigational problems. The exploratory factor analysis revealed four complaint intention factors. The four factors were labelled as electronic communication, switching behaviour, complaints to the retailer, and negative word-of-mouth. Negative word-of-mouth as private complaint action was the most relatively pertinent complaint intention, followed by switching intention and complaining to the retailer. Electronic WOM was the least pertinent complaint intention. More specifically, an ANOVA test was performed to determine the differences in emerging, young and older Millennials’ complaint behavioural intentions following a service failure in the online clothing retail context. The descriptive results indicated that the majority of the Millennial cohort would instead take action than no action. ANOVA’s were performed to determine the differences across Emerging, Young and Older Millennials’ complaint intentions following a service failure in the online clothing retail context. The ANOVA’s indicated that the three complaint intentions of electronic communication, switching, and complaining to retailers did not vary across the Millennials cohorts. This implies that the respective Millennial groups equally intended to take these individual complaint actions. Also, only word-of-mouth intention differed significantly across the Millennial groups. Older Millennials were less likely to contact family and/or friends in person or by text messaging on WhatsApp than Emerging and Young Millennials. The study makes a valuable contribution towards the consumer complaint behaviour literature and for consideration by online retailers, multi or omni-channel retailers, and marketers of clothing products. / Dissertation (MConsumer Science)--University of Pretoria, 2021. / Consumer Science / MConsumer Science (Clothing Management) / Unrestricted