Ultrasmall iron oxide nanoparticles have recently attracted much attention as T₁ (positive) contrast agents for magnetic resonance imaging to serve as safer alternatives to gadolinium-based T₁ contrast agents. This review will discuss several key aspects regarding this newly developed contrast agent, including synthetic strategies, parameters affecting T₁ signal (e.g., size, surface, and environment), and emerging applications. The integration with other imaging modalities will be discussed as well, such as dual T₁/T₂ imaging, positron emission tomography, and computed tomography. Finally, perspectives and outlook about the future development and concerns will be included.