Brain tissue comprises an extremely complex arrangement of cells that together make up the information-processing network enabling brain function. The connectivity of neurons underlies the unparalleled capabilities of our mind. Thus, mapping brain structure, which underlies brain function, has become a central focus in neuroscience. Electron microscopy provides extremely high resolution and comprehensive visualization of brain structure but requires correlative workflows to access molecular information. Light microscopy holds tremendous potential to analyze the ultrastructure of brain tissue together with its molecular makeup. However, conventional light microscopy (LM) has limited resolution (~ 200 nm laterally and ~ 1000 nm axially), far too coarse to distinguish neuronal structures in comprehensively labelled tissue and to precisely locate specific molecular players within sub-micrometer-sized structures, such as synapses. We have developed an optical imaging approach based on high-fidelity hydrogel expansion to visualize even the finest of neuronal structures, including axons and dendritic spines, when paired with comprehensive structural labeling and imaged with diffraction-limited, high-speed confocal microscopy, unveiling neuronal structure together with molecular information (Tavakoli et al., biorxiv, 2024). This enables deep-learning based, dense segmentation of neuronal structures and determining neuronal connectivity at single-synapse resolution, which we showcase in mouse cortex and hippocampus. Our technology will help shed light on brain structure, connectivity and molecular composition in a readily adoptable manner.