The CTAO is the next generation ground-based observatory for gamma-ray astronomy at very-high energies in the range from 20 GeV to 300 TeV. The CTAO’s detection of high-energy gamma rays is based on the imaging air Cherenkov technique (see How CTAO Works). Three classes of telescope are required to cover the full CTAO energy range (20 GeV to 300 TeV). For its core energy range (150 GeV to 5 TeV), the CTAO is planning up to 23 Medium-Sized Telescopes distributed over both array sites for the “Alpha Configuration“ (first construction phase). Up to four Large-Sized Telescopes and 37 Small-Sized Telescopes are planned to extend the energy range below 150 GeV and above 5 TeV, respectively.