  CMB-S4 -- the next-generation ground-based cosmic microwave background (CMB)
experiment - will significantly advance the sensitivity of CMB measurements and
improve our understanding of the origin and evolution of the universe. CMB-S4
will deploy large-aperture telescopes fielding hundreds of thousands of
detectors at millimeter wavelengths. We present the baseline optical design
concept of the large-aperture CMB-S4 telescopes, which consists of two optical
configurations: (i) a new off-axis, three-mirror, free-form anastigmatic design
and (ii) the existing coma-corrected crossed-Dragone design. We also present an
overview of the optical configuration of the array of silicon optics cameras
that will populate the focal plane with 85 diffraction-limited optics tubes
covering up to 9 degrees of field of view, up to $1.1 \, \rm mm$ in wavelength.
We describe the computational optimization methods that were put in place to
implement the families of designs described here and give a brief update on the
current status of the design effort.
