PACE is NASA's Plankton, Aerosol, Cloud, ocean Ecosystem mission, currently in the design phase of mission development. It is scheduled to launch in 2024, extending and improving NASA's over 20-year record of satellite observations of global ocean biology, aerosols (tiny particles suspended in the atmosphere), and clouds. PACE will advance the assessment of ocean health by measuring the distribution of phytoplankton, tiny plants and algae that sustain the marine food web. It will also continue systematic records of key atmospheric variables associated with air quality and Earth's climate. PACE has two fundamental science goals: To extend key systematic ocean color, aerosol, and cloud data records for Earth system and climate studies. To address new and emerging science questions using its advanced instruments, surpassing the capabilities of previous and current missions.