The Spanish Workers Statute (Royal Decree 1/1995 of the 24th March 1995) states that an employee is entitled to be paid on time. Where you are not paid on time you should issue proceedings to claim your wages. To do this your employer must owe you a minimum of two months wages.
Additionally, you can request the termination of your contract of employment if you wish. In this case you would need to issue two claims, one to claim the money owed and the other to terminate your contract. However you cannot leave your job until the Court has issued its Judgement, otherwise your employer could claim that you have left your job voluntarily.
If you find that you are a few months in arrears then your employer is obviously having serious financial difficulties and the best thing to do is to take proceedings to claim your wages and terminate your contract.
Your wages or your total remuneration, in cash and incentives, is payment to you for services rendered by you to your employer. Article 29.1 of the Workers’ Statute states that the period for regular wage payments should never be more than one month. You can also be paid daily, weekly or fortnightly. The place and method of payment is can be specified either in your own personal written contract, or a separate document, or possibly even verbally.
If your employer has not paid any wages for several months or it declares itself insolvent the employee must instigate two procedures in the Servicio de Mediación, Arbitraje y Conciliación (SMAC) of the autonomous region where you are working. You must exercise your right to make your claim within one year of the arrears arising and you can claim up to 12 months arrears of wages and it will be for you to prove the arrears.