The unexpected impact of an asteroid near Chelyabinsk, Russia, in 2013 attracted worldwide attention. More than 7,000 buildings were damaged and almost 1,500 people were injured. If an object less than 20 meters in diameter can cause such damage, what might larger ones do? We will introduce the impact problem and discuss various elements, including efforts to survey the population of threatening asteroids, governmental interest, impact effects, and what might be done to prevent such events by deflecting or disrupting objects. The scientific and engineering aspects of defending Earth from impacts also will be discussed, including the challenges of modeling deflection and/or disruption, the wide range of length and time scales involved, and the role of asteroid characteristics.