  [Abridged]. We present SEDs for single-age, single-metallicity stellar
populations (SSPs) covering the optical range at resolution 2.3A (FWHM). These
SEDs constitute our base models, as they combine scaled-solar isochrones with
MILES empirical stellar library, which follows the chemical evolution pattern
of the solar neighbourhood. The models rely as much as possible on empirical
ingredients, not just on the stellar spectra, but also on extensive photometric
libraries. The unprecedented stellar parameter coverage of MILES allowed us to
safely extend our optical SSP SED predictions from intermediate- to very-old
age regimes, and the metallicity coverage of the SSPs from super-solar to
[M/H]=-2.3. SSPs with such low metallicities are particularly useful for
globular cluster studies. Observed spectra can be studied by means of full
spectrum fitting or line-strengths. For the latter we propose a new Line Index
System (LIS) to avoid the intrinsic uncertainties associated with the popular
Lick/IDS system and provide more appropriate, uniform, spectral resolution.
Apart from constant resolution as function of wavelength the system is also
based on flux-calibrated spectra. Data can be analyzed at three different
resolutions: 5A, 8.4A and 14A (FWHM), which are appropriate for studying
globular cluster, low and intermediate-mass galaxies, and massive galaxies,
respectively. Polynomials to transform current Lick/IDS line index measurements
to the new system are provided. A web-page with a suite of on-line tools to
facilitate the handling and transformation of the spectra is available at
http://miles.iac.es.
