The controversy over “how free the trivalent silicon cation is” placed the emphasis on their spectroscopic and crystallographic characterisation. Those investigations contributed substantially to the understanding of the role of σ or π Lewis basic molecules in the chemistry of extremely potent Lewis acids, culminating in the discovery of innocent, that is non-coordinating, anions and an X-ray molecular structure. Synthetic uses of such reactive intermediates are however underrepresented yet a few recent contributions offer a glimpse of their true potential. This perspective provides a concise overview of known silylium ions, followed by examples of their uses as catalysts in C–F bond activation and C–C bond formation, namely challenging Diels–Alder reactions.