When a cruise boat stops at a port, thousands of tourists descend upon its city en masse - which might not be so great for those not coming from the boat.
While popular European destinations are already overrun during peak summer months, cruise ships add thousands of tourists entering a city in one day, congesting all the popular sights and creating long queues.
One handy tool to help you suss out how to spot the busiest periods is Avoid Crowds - a website that uses tourism data like cruise ship schedules and holidays that will tell you what the crowd would be like in any big city.
We had a look at some of the busiest port cities and when's the best time to avoid their cruise season.
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Barcelona
Cruise season: April to November, while October will be the busiest month followed by May.
Rome
Cruise season: There's an expected 8% increase in cruise passengers predicted for 2020, starting in May and ending in October. The end of the season is the busiest.
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Southampton
Cruise season: April to October, while the busiest month is September followed by May and August.
Venice
Cruise season: May until October, peaking in August.
READ: It's official - visiting Venice will cost a few extra euros for day-trippers
Santorini
Cruise season: March to October, with June and September as the bearing the brunt of the crowds.
Dubrovnik
Cruise season: May to October, with the last month being the busiest.
Bergen
Cruise season: High season is between May and September, although the year-round destination gets cruise ships from January to November.
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Valletta
Cruise season: April to October is peak season, with May and October as the busiest times.
Kotor
Cruise season: September to October, with the last month of the short season seeing the most cruise passengers.
Copenhagen
Cruise season: May to September is this city's cruise season, peaking in July and August.
Lisbon
Cruising season: Starts in April all the way to November.
WATCH: Two city itineraries for a long layover in Lisbon
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