Today, the 29th June 2015, the currency of Greece is still the Euro. It is advised that you take other forms of payment with you, as well as cash. Make sure you've got your debit card and/or credit card. However, there's a possibility that Greek banking services (including service of ATMs and the processing of credit card payments) could become limited at short notice.
You need to make sure you have enough in Euros to cover any emergencies.
There's also a lot of strike action going on over there, so there can be disruption to public transport in and out of Greece. There's also a lot of demonstrations going down, especially in Athens. The government have a dedicated page regarding planned major strikes and demos, which you can see here.
More worryingly, there's a 'general threat' of terrorism and acts of political violence, and again, there's a dedicated page where you can keep informed about all that, here.
If you're in Greece and you need to call the emergency services, the number is 112. If you have your UK mobile with you, you can dial 999 and it will automatically put you through to the Greek emergency services.