Network professionals know that distributed denial-of-service attacks are an ever-growing danger. The recent assault on Twitter is just the latest evidence. Using a mushrooming array of advanced tools, including pay-per-use services and mobile devices, attackers are taking down websites, DNS and email servers, often using these tools to destroy a company's online revenue, customer service and brand reputation. But the technology is only half the story. The thinking that shapes attacks — an evolving blend of careful planning, probing and improvisation—is often the difference between duds and strikes that leave victims begging for mercy.