Despite evidence indicating the significant influence of online reviews on purchase decisions, even after taking into account a product's average rating (Vana and Lambrecht 2021), the underlying factors responsible for this effect and the broader impact of reviews on consumer decision-making remain uncertain. This study uses click-stream data from a major online retailer to explore how negative reviews affect consumer search and purchase decisions. Leveraging exogenous variation created by the display of online reviews sorted by recency, the authors find that negative reviews significantly reduce a product's purchase probability because they (1) contrast with the often-high average product rating, (2) decrease the probability that consumers continue browsing for information about the focal product, (3) increase the probability of visiting the page of substitute products, and (4) increase the probability of viewing reviews about substitute products. Importantly, these effects apply to utilitarian products but not hedonic products and when reviews pertain to product functionality or customer service but not to taste-related factors. The authors estimate a product's vulnerability to negative reviews along two dimensions - purchase and search probability for substitutes - and display these effects on a two-dimensional map.