Ultrasmall　iron　oxide　nanoparticles　have　recently　attracted　much　attention　as　T-1　(positive)　contrast　agents　for　magnetic　resonance　imaging　to　serve　as　safer　alternatives　to　gadolinium-based　T-1　contrast　agents.　This　review　will　discuss　several　key　aspects　regarding　this　newly　developed　contrast　agent,　including　synthetic　strategies,　parameters　affecting　T-1　signal　(e.g.,　size,　surface,　and　environment),　and　emerging　applications.　The　integration　with　other　imaging　modalities　will　be　discussed　as　well,　such　as　dual　T-1/T-2　imaging,　positron　emission　tomography,　and　computed　tomography.　Finally,　perspectives　and　outlook　about　the　future　development　and　concerns　will　be　included.