HabEx is one of four candidate flagship missions being studied in detail by NASA, to be submitted for consideration to the 2020 Decadal Survey in Astronomy and Astrophysics for possible launch in the 2030s. It will be optimized for direct imaging and spectroscopy of potentially habitable exoplanets, and will also enable a wide range of general astrophysics science. HabEx aims to fully characterize planetary systems around nearby solar-type stars for the first time, including rocky planets, possible water worlds, gas giants, ice giants, and faint circumstellar debris disks. In particular, it will explore our nearest neighbors and search for signs of habitability and biosignatures in the atmospheres of rocky planets in the habitable zones of their parent stars. Such high spatial resolution, high contrast observations require a large (roughly greater than 3.5m), stable, and diffraction-limited optical space telescope. Such a telescope also opens up unique capabilities for studying the formation and evolution of stars and galaxies. We present some preliminary science objectives identified for HabEx by our Science and Technology Definition Team (STDT), together with a first look at the key challenges and design trades ahead.