One of the key challenges during process development and production of biologics is the timely determination of essential product attributes. Rapid feedback of product-relevant characteristics simplifies and accelerates process development and scale-up, then helps ensure product yield and quality during full-scale manufacturing. We present how the process analytical technology Real-Time Multi-Angle Light Scattering (RT-MALS) is used to optimize process development of viral vectors. RT-MALS determines the biophysical properties size, molar mass and concentration of virions or nanoparticles. The application of RT-MALS in downstream processing of viral vectors is shown and discussed.