Clayton Christensen's landmark work introduces the concept of disruptive innovation — one of the most influential and widely misused ideas in business strategy over the past three decades. Based on detailed historical case studies of the disk drive, steel, and excavator industries, Christensen demonstrates a paradox: well-managed companies following best practices of listening to customers, investing in high-margin products, and improving established technologies are systematically vulnerable to displacement by inferior entrants who begin by serving overlooked or low-end customers with simpler, cheaper, less capable products. The innovation dilemma is that the rational, customer-responsive decisions that sustain market leadership in the short run — ignoring low-margin customer segments, focusing investment on products current customers want — simultaneously make incumbents blind to the trajectory of disruptive technologies that eventually improve enough to take the mainstream market. Christensen identifies the conditions under which disruption is likely, the organizational capabilities needed to respond, and why spinning off separate units with different processes and values is often the only reliable defense. The theory has been widely applied to industries from newspapers and music to taxis and banking, though Christensen himself spent subsequent years refining which innovations actually qualified as "disruptive" versus merely competitive. The original book remains the clearest and most empirically grounded statement of his framework, and its historical examples are as illuminating as ever.