ELODIE is a high-resolution spectrograph at the 1.93 m telescope of the Observatoire de Haute-Provence, France. It was commissioned in 1993 and is designed for accurate radial velocity measurements, crucial for detecting brown dwarfs and giant planets around nearby stars. ELODIE uses a 42000 resolution echelle grating and a combination of a prism and grism as cross-disperser to record a spectrum from 3906 Å to 6811 Å on a 1024×1024 CCD in a single exposure. An automatic data reduction system processes the spectra and computes cross-correlation functions. The instrument's design and data reduction algorithms are described, allowing radial velocity measurements with an accuracy better than 15 m s⁻¹ for 9th magnitude stars in less than 30 minutes. It can also measure velocities of 16th magnitude stars with an accuracy of about 1 km s⁻¹. For classical spectroscopic studies, 9th magnitude stars can be observed in one hour. ELODIE is part of an accelerometer instrument designed by Connes and is not described here. The instrument's design includes a "white pupil" mounting, a 408×102 mm echelle grating with a tanθ=4 blaze angle, and a 300 mm focal length camera with an f/3 aperture ratio. The optical layout includes a fibre-fed system, a Czerny-Turner mounting, and a combination of a prism and grism for cross-dispersion. The instrument uses a 1024×1024 CCD with 24 μm pixels, cooled to 183 K, and has four gain settings. The data reduction process includes flat-field correction, wavelength calibration with a thorium lamp, and cross-correlation functions for radial velocity measurements. The instrument's performance is validated by its ability to achieve high accuracy and stability, making it suitable for both long-term and short-term observations. ELODIE is also used for other spectroscopic studies, including rotational velocities of late-spectral type stars and stellar metallicities. The instrument's design and data reduction algorithms are described in detail, highlighting its efficiency and accuracy in radial velocity measurements.ELODIE is a high-resolution spectrograph at the 1.93 m telescope of the Observatoire de Haute-Provence, France. It was commissioned in 1993 and is designed for accurate radial velocity measurements, crucial for detecting brown dwarfs and giant planets around nearby stars. ELODIE uses a 42000 resolution echelle grating and a combination of a prism and grism as cross-disperser to record a spectrum from 3906 Å to 6811 Å on a 1024×1024 CCD in a single exposure. An automatic data reduction system processes the spectra and computes cross-correlation functions. The instrument's design and data reduction algorithms are described, allowing radial velocity measurements with an accuracy better than 15 m s⁻¹ for 9th magnitude stars in less than 30 minutes. It can also measure velocities of 16th magnitude stars with an accuracy of about 1 km s⁻¹. For classical spectroscopic studies, 9th magnitude stars can be observed in one hour. ELODIE is part of an accelerometer instrument designed by Connes and is not described here. The instrument's design includes a "white pupil" mounting, a 408×102 mm echelle grating with a tanθ=4 blaze angle, and a 300 mm focal length camera with an f/3 aperture ratio. The optical layout includes a fibre-fed system, a Czerny-Turner mounting, and a combination of a prism and grism for cross-dispersion. The instrument uses a 1024×1024 CCD with 24 μm pixels, cooled to 183 K, and has four gain settings. The data reduction process includes flat-field correction, wavelength calibration with a thorium lamp, and cross-correlation functions for radial velocity measurements. The instrument's performance is validated by its ability to achieve high accuracy and stability, making it suitable for both long-term and short-term observations. ELODIE is also used for other spectroscopic studies, including rotational velocities of late-spectral type stars and stellar metallicities. The instrument's design and data reduction algorithms are described in detail, highlighting its efficiency and accuracy in radial velocity measurements.