The interferometers being planned for second generation LIGO promise an order of magnitude increase in broadband strain sensitivity-with the corresponding cubic increase in detection volume-and an extension of the observation band to lower frequencies. In addition, one of the interferometers may be designed for narrowband performance, giving further improved sensitivity over roughly an octave band above a few hundred Hertz. This article discusses the physics and technology of these new interferometer designs, and presents their projected sensitivity spectra.