This paper presents a droplet-based microfluidic technology for high-throughput screening of single mammalian cells. The system enables the encapsulation of single cells and reagents in independent aqueous microdroplets (1 pL to 10 nL) dispersed in an immiscible carrier oil, allowing for the digital manipulation of these reactors at high throughput. The authors validate a full droplet screening workflow by conducting a droplet-based cytotoxicity screen. They first developed a droplet viability assay to quantitatively score cell viability and growth within intact droplets. Next, they demonstrated the high viability of encapsulated human monocytic U937 cells over 4 days. Finally, they developed an optically-coded droplet library to identify droplet composition during the assay read-out. Using the integrated droplet technology, they screened a drug library for its cytotoxic effect against U937 cells. The droplet-based microfluidic approach provides increased throughput, reduced sample volumes, and single-cell analysis capabilities. It uses a two-phase system where each assay is compartmentalized in an aqueous microdroplet surrounded by an immiscible oil. The advantages of this technique include physical and chemical isolation of droplets, fast and efficient mixing of reagents, high-throughput digital manipulation of droplets, stable droplet incubation off-chip, and the absence of moving parts. This technology is particularly suitable for working with cells of limited availability, such as stem cells or primary cells from patients. The authors developed an integrated on-chip viability assay that allows for the analysis of cell viability within intact droplets. This assay includes five different droplet modules: generation and reinjection, merging, mixing, delay line, and detection. The assay was used to demonstrate the survival of encapsulated cells over several days. The system was also used to screen a drug library for its cytotoxic effect on U937 cells. The results showed that the droplet workflow is generic and can be used with different types of libraries, fluorescent assay read-outs, or additional droplet manipulation modules. The study demonstrates the successful integration of different droplet modules into a single on-chip viability assay with high specificity and accuracy. The system allows for the encapsulation and storage of cells for up to 4 days with high survival rates. The authors also tested the system's ability to accurately discriminate different cell populations and showed that the on-chip viability assay is highly sensitive and specific. The results showed that the droplet-based technology is a robust and versatile platform for high-throughput single-cell screening, with potential applications in various fields including combinatorial screening and small sample analysis. The system is also well-suited for analysis of small samples of medical relevance.This paper presents a droplet-based microfluidic technology for high-throughput screening of single mammalian cells. The system enables the encapsulation of single cells and reagents in independent aqueous microdroplets (1 pL to 10 nL) dispersed in an immiscible carrier oil, allowing for the digital manipulation of these reactors at high throughput. The authors validate a full droplet screening workflow by conducting a droplet-based cytotoxicity screen. They first developed a droplet viability assay to quantitatively score cell viability and growth within intact droplets. Next, they demonstrated the high viability of encapsulated human monocytic U937 cells over 4 days. Finally, they developed an optically-coded droplet library to identify droplet composition during the assay read-out. Using the integrated droplet technology, they screened a drug library for its cytotoxic effect against U937 cells. The droplet-based microfluidic approach provides increased throughput, reduced sample volumes, and single-cell analysis capabilities. It uses a two-phase system where each assay is compartmentalized in an aqueous microdroplet surrounded by an immiscible oil. The advantages of this technique include physical and chemical isolation of droplets, fast and efficient mixing of reagents, high-throughput digital manipulation of droplets, stable droplet incubation off-chip, and the absence of moving parts. This technology is particularly suitable for working with cells of limited availability, such as stem cells or primary cells from patients. The authors developed an integrated on-chip viability assay that allows for the analysis of cell viability within intact droplets. This assay includes five different droplet modules: generation and reinjection, merging, mixing, delay line, and detection. The assay was used to demonstrate the survival of encapsulated cells over several days. The system was also used to screen a drug library for its cytotoxic effect on U937 cells. The results showed that the droplet workflow is generic and can be used with different types of libraries, fluorescent assay read-outs, or additional droplet manipulation modules. The study demonstrates the successful integration of different droplet modules into a single on-chip viability assay with high specificity and accuracy. The system allows for the encapsulation and storage of cells for up to 4 days with high survival rates. The authors also tested the system's ability to accurately discriminate different cell populations and showed that the on-chip viability assay is highly sensitive and specific. The results showed that the droplet-based technology is a robust and versatile platform for high-throughput single-cell screening, with potential applications in various fields including combinatorial screening and small sample analysis. The system is also well-suited for analysis of small samples of medical relevance.