Montenegro's National Tourism Organisation says the vast majority of hotels, restaurants, bars, petrol stations and shops accept cards, and that ATMs are available in every city. That makes cards a strong default, but not a reason to travel with an empty cash pocket.
Let the landscape divide the wallet
Card-ready does not mean card-guaranteed
A terminal can lose its connection, a seasonal business can set a minimum or a small family restaurant can simply prefer cash. Ask before ordering when the last ATM is several bends behind you.
Use an ATM attached to a real bank
Montenegro's cities have a substantial ATM network. A machine at an established bank branch gives you a clearer operator and a better place to seek help than an anonymous tourist-area terminal. The screen should reveal its own access fee before the final confirmation.
Keep the conversion in euros
If the ATM or card terminal offers to charge your home currency, compare carefully. Declining that conversion and continuing in EUR usually leaves the exchange to your own issuer or card network, whose rate may be better.
One planned withdrawal keeps fees in proportion
A fixed ATM fee is most expensive when repeated for tiny amounts. Withdraw enough for the cash-dependent part of the route, split it between a daily pocket and a separate reserve, then return to the card in larger businesses.