As the most intensively investigated ferroelectric polymers, poly(vinylidene fluoride) and its co-/ter-polymers enable major breakthroughs in a wide range of applications. Since defects play a vital role in tuning a spectrum of physical properties of poly(vinylidene fluoride)-based ferroelectric polymers, defect engineering has become an ingenious and robust strategy in the design of high-performance ferroelectric polymers. In this Review, we summarize the physical insights into the role of defects induced by various monomers at the molecular level on the physical properties and the structure–property relationship of defect-modified ferroelectric polymers. We focus on the fundamentals of the different structural defects on tailoring the dielectric, ferroelectric, electromechanical, and electrocaloric properties, along with the device performance enhancement in capacitors, actuators, and solid-state cooling. The influence of defects on the electric field dependence of the electrostriction and electrocaloric response is highlighted. The role of chiral defects in driving the emergent relaxor properties and morphotropic phase boundary behavior of ferroelectric polymers is discussed. Finally, we offer insightful perspectives on the challenges and opportunities in this rapidly evolving field. The underlying mechanisms revealed in the article are anticipated to guide future fundamental and applied studies of ferroelectric polymers that capitalize on defect engineering for electronic and energy applications.