My tenant has stopped paying rent - what's the first thing I should do?

Serve a formal written demand giving 30 days to pay, ideally through a Notary Public or registered mail so it is provable. Under Article 25(1) of Law 33/2008, if the tenant still hasn't paid 30 days after this notice, you gain grounds to file for eviction and rent recovery at the Rental Dispute Centre. Keep every payment record and message.