  We present a conceptual design for the atmospheric dispersion corrector (ADC)
for TMT's Infrared Imaging Spectrograph (IRIS). The severe requirements of this
ADC are reviewed, as are limitations to observing caused by uncorrectable
atmospheric effects. The requirement of residual dispersion less than 1
milliarcsecond can be met with certain glass combinations. The design decisions
are discussed and the performance of the design ADC is described. Alternative
options and their performance tradeoffs are also presented.
