If you are residente in Spain - basically, those who spend more than 183 days a year here, or whose business is based here (if in doubt, check with your asesor fiscal) - and you have capital and/or property outside Spain worth more than 50,000 euros, then you need to declare it to the Spanish tax authorities.
The new requirements, which were brought in as part of the anti-fraud law 7/2012, mean that you are obliged to report assets for 2012 by 30 April 2013, and in future years at the end of the first trimester (31 March). The BOE (Boletin Oficial del Estado) was published on 31 January 2013, under the official title "Declaracion Informativa Sobre Bienes y Derechos Situados en el Extranjero".
The foreign assets which must be declared - with their value (or balance over last three months for bank accounts) stated as of 31 December 2012 - are:
* bank accounts
* stocks, share, securities, life insurance policies, income generated by lending money, rights or other assets to foreign entities
* property and rights over property
If you are the owner, beneficiary or authorised signatory of any such assets of which the total in any category exceeeds 50,000 euros, then you must declare the asset to the Agencia Tributaria. You also need to quote the acquisition value of any property you own. If you sold an asset, for example property or shares, you need to declare the value at date of sale.
You won't need to report your assets in future years, unless their value increases by more than 20,000 euros.
As it is in dire need of raising funds, the AEAT will be clamping down harder than ever on those who try to escape this declaration, with fines starting at 5,000 euro.
The form which you need is Model 720, which has to be submitted electronically, so you need a certificado digital (Spanish can use their DNIe - electronic DNI). Please bear in mind that these are new regulations - new for taxpayers, new for asesores, and new for our friends at the AEAT. So allow yourself plenty of time to find out exactly if, and what, you need to declare, and if so exactly what information you need. Ideally, talk to your asesor fiscal.
Other useful links: AEAT (Agencia Tributaria) Campaña de Declaraciones Informativos Deccember 2012. Pages 50-59 explain what information is needed.
Relevant BOE (Boletin Oficial del Estado) with a section on Modelo 720, explaining what you need to write in each box (numbers or letters).