Catalonia is an autonomous region in north-east Spain with a distinct history dating back almost 1,000 years.
The wealthy region has about 7.5 million people, with their own language, parliament, flag and anthem. Catalonia also has its own police force and controls some of its public services.
Why the controversy?
For many years Catalan nationalists have complained that their region sends too much money to poorer parts of Spain.
Their budget and taxes are controlled by Madrid.
They also say Spain's changes to their autonomous status in 2010 undermined their distinctive Catalan identity.
In a referendum on 1 October, declared illegal by Spain's Constitutional Court, about 90% of Catalan voters had backed independence. But turnout was only 43%.
There were clashes when Spanish national police tried to prevent people voting.
The Catalan parliament then voted for independence on 27 October. At the same time Madrid moved to impose direct control by invoking Article 155 of the constitution - a first for Spain.
What is Madrid doing?
The Spanish government has fired the Catalan leaders, dissolved parliament and called regional elections for 21 December.
Sacked Catalan President Carles Puigdemont remains defiant and has urged civil servants to disobey Madrid.
Why does the crisis matter?
The crisis does not look set to degenerate into armed conflict, but it could damage the region and Spain as a whole economically, bringing new instability to the eurozone.
It is being watched nervously by other countries with secessionist movements in Europe.
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