Transmission electron microscopy (TEM) is arguably the most common method used to study the size evolution of individual polymer assemblies. 7, However, conventional TEM provides only 'snapshots' of the distribution of particle sizes during growth, lacking the capability to track the transformations of individual nanoparticles. Exciting recent advances in liquid cell TEM have opened a route to monitor the liquid phase reaction in-situ with millisecond to second temporal resolution, albeit at the expense of significant instru- mental complexity. Solution-phase atomic force microscopy (AFM) has also been applied to study the mechanism of interfacial seeded-growth of 1D micellar nanoparticles. However, tip-induced nanofiber fragmentation has been reported as a drawback; creating new active interfaces and affecting kinetics. Fluorescence microscopy offers a perhaps less invasive alternative, but at the expense of spatial resolution: For example, confocal laser scanning microscopy (CLSM) has been used to visualize 1D fiber growth at 100 ms temporal resolution with 120 nm lateral spatial resolution. Super-resolution fluorescence microscopy techniques can improve these limits with, for instance, 76 nm precision and 15 ms temporal resolution of the growth of 1D fiber. Although providing easy access to nanoparticle kinetics, fluorescence microscopy's significant drawback is the requirement for extrinsic labels; given the relative size of a fluorophore to component monomers, it is perhaps unsurprising that impacts on the CDSA kinetics are reported. Label-free techniques such as phase contrast or differential interference contrast microscopy can circumvent this issue but are typically limited in detection sensitivity. To address the need for an in-situ non-invasive method of characterizing living CDSA kinetics with high spatio-temporal single-particle resolution, here we apply interferometric scattering (iSCAT) microscopy to track CDSA in individual polymer particles. iSCAT is an intrinsically label-free technique capable of single-molecule resolution. To date, iSCAT has mainly been used to study biological systems, including the mass measurement of individual proteins, the formation of lipid membranes, and biological diffusion using single-particle tracking of metal nanoparticles. iSCAT has also been effectively utilized to monitor protein aggregation and actin polymerization processes. Given the similarities shared between actin polymerization and 1D CDSA fiber formation, we reasoned that iSCAT might also be productively applied to the study of CDSA.