This document outlines the high-level science requirements of the Zwicky Transient Facility (ZTF), an optical time-domain survey to be based at Palomar Observatory. ZTF is the successor to the Palomar Transient Factory (PTF; 2009–2012) and Intermediate Palomar Transient Factory (iPTF; 2013–present) surveys. PTF and iPTF used a secondhand camera on the Palomar 48-inch Schmidt telescope to survey the sky for astronomical transients and variables. ZTF will instead use a purpose-built survey camera on the same telescope to achieve more than an order of magnitude improvement in survey speed relative to PTF.