The use of brand extensions has become fundamental to the business model of most luxury brands. Many traditional luxury brands such as Louis Vuitton or Chanel have expanded into traditional luxury sectors beyond their core business. Some brands such as Armani or Prada even crossed boundaries to nontraditional lifestyle segments to pursue new business opportunities. Given the high practical relevance of brand extensions for luxury brands and the importance to understand the success factors for their extendibility and potential backward effects on the parent brand, surprisingly little research has addressed these issues for luxury brands in comparison to nonluxury brands. The current research reveals extension-related differences between luxury and nonluxury brands by simultaneously analyzing key dimensions of parent brand value, fit, and extension category involvement on the consumer's attitude toward the brand extension, which in turn influences the postextension image of the parent brand. Results of a structural equation model based on a survey among 492 participants show that the predominant driver of brand extension success for both luxury and nonluxury brands is overall extension fit, followed by the consumer's involvement in the extension category. The influence of functional value of the parent brand on the extension evaluation is more important for nonluxury brands. The hedonic value of the parent brand is found to be of relevance only in case of luxury brands. Moreover, a reciprocal spillover effect between the extension evaluation and the parent brand evaluation is observed. The degree of luxuriousness of the parent brand moderates this relationship. This effect is weaker for luxury brands.