The concept of disruptive innovation or disruptive technologies as Clayton M. Christensen first introduced it in 1995. His article Disruptive Technologies: Catching the Wave, which he co-authored with Joseph Bower, was targetted at executives responsible for purchasing decisions in companies and not executives in research roles. Christensen further explored the concept of disruption in his book The Innovator's Dilemma. However, it was not until his follow up book with Michael E. Raynor, The Innovator's Solution, that Christensen replaced the term disruptive technology with disruptive innovation. The reason for this was that he realised that it was not the technology that, in essence, was the disruptive force of innovation; instead, it was the business model that was the root cause of the disruption. Once this is understood, innovators or entrepreneurs can focus their energy of the strategy of innovation on playing where their competitors are at their most uncomfortable, enforcing 'radical change' within an industry. Something incumbents do not wish, that is a radical change.