Significant advancements in upstream biomanufacturing methods for cell and gene therapies have been achieved in recent years. However, final yields continue to be impacted by losses in downstream steps. Interferometric Light Microscopy (ILM) represents a fast and cost-effective method for process development teams to obtain size and concentration data within seconds. This presentation will cover the technical basis of ILM as it compares to other methods for monitoring nanoparticles. Experimental studies developing ILM utilization in characterizing virus preparations, detecting virus breakage and aggregation, and monitoring lipid nanoparticle preparations will be discussed.