The HERMES high-resolution spectrograph project aims at exploiting the speciﬁc potential of small but ﬂexible telescopes in observational astrophysics. The optimised optical design of the spectrograph is based on the well-proven concept of white-pupil beam folding for high-resolution spectroscopy. In this contribution we present the complete project, including the spectrograph design and procurement details, the telescope adaptor and calibration unit, the detector system, as well as the optimised data-reduction pipeline. We present a detailed performance analysis to show that the spectrograph performs as speciﬁed both in optical quality and in total eﬃciency. With a spectral resolution of 85000 (63000 for the low-resolution ﬁbre), a spectral coverage from 377 to 900 nm in a single exposure and a peak eﬃciency of 28%, HERMES proves to be an ideal instrument for building up time series of high-quality data of variable (stellar) phenomena.