Lipid nanoparticles (LNPs) have emerged as a powerful platform for nucleic acid delivery, particularly for mRNA vaccines. However, a major bottleneck in this field is the release of mRNA-LNPs from endosomes into the cytosol, where they can function. The mechanisms of endosomal escape are poorly understood, and there is no consensus on the compartment of escape or the causes of inefficient escape. This review discusses the current knowledge of endosomal escape mechanisms and methods to study this process. It highlights the limitations and challenges of existing methods and proposes the development of robust, quantitative high-throughput techniques to improve our understanding and enhance RNA-LNP designs. Better understanding of endosomal escape will enable the development of more effective RNA-LNP therapeutics, unlocking new therapeutic modalities.Lipid nanoparticles (LNPs) have emerged as a powerful platform for nucleic acid delivery, particularly for mRNA vaccines. However, a major bottleneck in this field is the release of mRNA-LNPs from endosomes into the cytosol, where they can function. The mechanisms of endosomal escape are poorly understood, and there is no consensus on the compartment of escape or the causes of inefficient escape. This review discusses the current knowledge of endosomal escape mechanisms and methods to study this process. It highlights the limitations and challenges of existing methods and proposes the development of robust, quantitative high-throughput techniques to improve our understanding and enhance RNA-LNP designs. Better understanding of endosomal escape will enable the development of more effective RNA-LNP therapeutics, unlocking new therapeutic modalities.