The development of voltage-sensitive fluorescent probes has enabled the direct optical measurement of electrical activity in biological tissues. By measuring fluorescence lifetime instead of intensity, it is possible to avoid motion artifacts, a key limitation in conventional fluorescence imaging of dynamic, contracting samples like heart tissue. Lifetime measurements also capture sub-threshold events, providing valuable insights for studying arrhythmias and other electrical phenomena in cardiac cells. Recent advances in single-photon avalanche diode (SPAD) cameras, offering improved detection efficiencies, fill factors, and pixel densities, along with sophisticated computational techniques such as Bayesian inversion and neural networks, have opened up new frontiers for fluorescence lifetime imaging microscopy (FLIM). FLIM is a powerful tool in biomedical imaging, offering a unique view into cellular environments, molecular interactions, and tissue physiology. However, conventional FLIM techniques face trade-offs between spatial and temporal resolution, with most current systems able to achieve either high spatial resolution or high temporal resolution, but not both simultaneously. For instance, leading commercial FLIM systems achieve megapixel resolution but require up to 2 seconds for image acquisition, which is impractical for live tissue imaging. To address these limitations, we have recently developed a high-speed widefield FLIM system using a dual-gated, 500×500 SPAD array (SwissSPAD3), capturing images at a rate of 192 frames per second. By synchronizing this SPAD array with a high-resolution sCMOS camera and applying advanced upsampling algorithms, we achieve effective megapixel resolution with enhanced temporal resolution. This PhD project aims to further develop, validate, and apply this FLIM system to study cardiac contractility and cardiotoxicity in functional assays. Specifically, we will improve the microscope setup to combine high spatial resolution from a megapixel intensity camera with the high temporal resolution from the SPAD array, achieving picosecond time resolution at 500-800 Hz over fields of view ranging from 0.5 to 1 mm. These advancements will enable FLIM imaging with an unprecedented combination of speed and resolution, possible only due to the latest SPAD technology. The project will then apply this FLIM system to image human induced pluripotent stem cell (iPSC)-derived cardiomyocytes stained with voltage- and calcium-sensitive dyes. By monitoring fluorescence lifetime changes during cellular contractions, we will visualize the propagation of action potentials and calcium waves, capturing spontaneous electrical activity with minimal motion artefacts. These lifetime-based images will be compared with traditional intensity-based images to quantify improvements in motion artefact robustness and sub-threshold event sensitivity. In collaboration with our industrial partner, Clyde Biosciences, we aim to significantly advance cardiotoxicity assays and imaging-based functional contractility assays. Current protocols employed by Clyde Biosciences analyze cell motion and contractility in label-free, monolayer cultures, measuring electrical and mechanical parameters indirectly. Our FLIM system, by contrast, will directly measure electrical activity, overcoming limitations in throughput, sensitivity, and motion artifact resiliance. This project proposes a transformative approach that treats natural movement as a source of meaningful data, not an artifact, allowing for real-time voltage imaging during contractile activity. This project establishes and showcases the potential for next-generation FLIM systems capable of high-resolution, high-speed imaging, promising improved sensitivity in biological studies and paving the way for novel applications such as cardiotoxicity screening, regenerative medicine, and beyond.