The Advanced LIGO gravitational wave detectors are second-generation instruments designed and built for the two LIGO observatories in Hanford, WA, and Livingston, LA. These instruments are specialized versions of a Michelson interferometer with 4 km long arms, featuring Fabry-Perot cavities and power recycling to increase interaction time with gravitational waves and effective laser power, respectively. Signal recycling has been added to improve the frequency response, particularly in the most sensitive region around 100 Hz, where the design strain sensitivity is a factor of 10 better than initial LIGO. The low-frequency end of the sensitivity band has been extended from 40 Hz to 10 Hz. All interferometer components have been replaced with improved technologies to achieve these gains, including better seismic isolation and test mass suspensions. Higher laser power, larger test masses, and improved mirror coatings contribute to the improved sensitivity at mid- and high-frequencies. Data collecting runs with these new instruments are planned to begin in mid-2015. The Advanced LIGO detectors are designed to provide a factor of 10 increase in strain sensitivity over a broad frequency band and to extend the low end of the band to 10 Hz, significantly increasing the number of potential astrophysical sources detectable. The interferometer configuration includes a Michelson interferometer with Fabry-Perot resonant cavities in each arm, power recycling, and signal recycling to enhance the frequency response. The design considerations for the arm cavity, recycling cavity, gravitational wave readout, and high-power effects are detailed, along with the strain sensitivity and various modes of operation. The detector subsystems, including the laser source, input optics, core optics, suspensions, seismic isolation, and thermal compensation, are described in detail, highlighting their critical components and performance requirements.The Advanced LIGO gravitational wave detectors are second-generation instruments designed and built for the two LIGO observatories in Hanford, WA, and Livingston, LA. These instruments are specialized versions of a Michelson interferometer with 4 km long arms, featuring Fabry-Perot cavities and power recycling to increase interaction time with gravitational waves and effective laser power, respectively. Signal recycling has been added to improve the frequency response, particularly in the most sensitive region around 100 Hz, where the design strain sensitivity is a factor of 10 better than initial LIGO. The low-frequency end of the sensitivity band has been extended from 40 Hz to 10 Hz. All interferometer components have been replaced with improved technologies to achieve these gains, including better seismic isolation and test mass suspensions. Higher laser power, larger test masses, and improved mirror coatings contribute to the improved sensitivity at mid- and high-frequencies. Data collecting runs with these new instruments are planned to begin in mid-2015. The Advanced LIGO detectors are designed to provide a factor of 10 increase in strain sensitivity over a broad frequency band and to extend the low end of the band to 10 Hz, significantly increasing the number of potential astrophysical sources detectable. The interferometer configuration includes a Michelson interferometer with Fabry-Perot resonant cavities in each arm, power recycling, and signal recycling to enhance the frequency response. The design considerations for the arm cavity, recycling cavity, gravitational wave readout, and high-power effects are detailed, along with the strain sensitivity and various modes of operation. The detector subsystems, including the laser source, input optics, core optics, suspensions, seismic isolation, and thermal compensation, are described in detail, highlighting their critical components and performance requirements.