The world-class 4.2m William Herschel telescope (WHT), with its comprehensive instrument suite at the superb site of the Observatorio del Roque de los  Muchachos (ORM), stood as the reference optical observational facility behind the dramatic rise in Spanish astronomy over the last 25 years.

Now that the Spanish community has access to 8-10-m-class telescopes (VLT, GTC), the role of the WHT is evolving towards offering what larger telescopes cannot large field-of-view (FoV) instrumentation. In response to calls from ASTRONET and other European advisory bodies, the Isaac Newton Group (ING) partners have formed a European consortium to design and build an advanced high-multiplex optical spectrograph for the WHT named WEAVE.

This facility comprises a new 2 degree FoV prime-focus corrector, a 1000-multiplex fiber positioner, a number of individually-deployable mini-integral field units (MIFU) and a large single integral field unit (LIFU).

The IFUs and the MOS fibers feed a dual-beam, high-throughput spectrograph that delivers full optical wavelength coverage in a single exposure at a spectral resolution of 5000, or a reduced wavelength coverage in each arm at a resolution of20.000. With approximately 70% of the WHT time devoted to surveys, in 5 years WEAVE will deliver close to 15 million high quality spectra of astronomical objects.

WEAVE data will provide essential astrophysical information to key European Northern Hemisphere programs like ESAs Gaia satellite as well as galaxy evolution and cosmology probes such as LOFAR.WEAVE is being developed by a large European collaboration involving the three ING partners (the UK, the Netherlands, and Spain) and France. The instrument passed its Preliminary Design Review in March 2013.

 The Final Design phase started in May 2013, and it is expected to finish in autumn2014. The construction and assembly phases of the instrument will cover the period from 2015 to 2017, and it is expected that the instrument will be commissioned in the second half of 2017.Funding for WEAVE has been secured from the UK (5m), from the Netherlands (3.5m), from France (3m), and from the ING itself (3m). The commitment of Spain is now required. The Instituto de Astrofísica de Canarias plans to lead essential WEAVE work packages during the design and construction phases of the instrument. Three main areas to be led by Spain have been identified and these are based on the interests and capabilities of Spanish industry, on the instrumental expertise at the IAC, and on our goal to provide the Spanish astronomical community with a competitive edge to produce world-class science using WEAVE. The Spanish contribution to WEAVE during the period 2014-2017 will focus on the collaboration in the design and development of: the Prime Focus Corrector system (PFC), the prime focus and spectrograph control systems, and the software required for the advanced processing of the science data. The total Spanish investment in WEAVE is estimated to be about 3m. This corresponds to about 17% of the total cost or the instrument, a fraction similar to the Spanish contribution to the ING operational budget.