Baroness Thatcher was the UK's first and only female prime minister.
She died peacefully, aged 87, on Monday the 8th of April 2013.
From the start of her time as prime minister Margaret Thatcher divided opinion.
When she came to power in 1979 she wanted to make big changes to how the country was run - and she transformed Britain.
One of her biggest changes was that she wanted private companies and people, and not officials, to run government-owned things like British Gas and BT (British Telecom). She believed that private companies would run the services better.
Thousands of ordinary people were able to buy their council houses, giving them more of a stake in society.
Under her rule, the City of London became one of the world's most successful centres for banking and business.
The PM became a hero to many when in 1982 she sent British Troops to defend the Falklands. The tiny islands eight thousand miles away in the South Atlantic sea had been invaded by Argentina. Although hundreds of lives were lost, the victory was celebrated on the streets back home.
But though she had many supporters, some of the changes she introduced were very unpopular and there were protests and riots on the streets.
Protests across the country
In 1984 thousands of miners protested for a whole year against her plans to shut down lots of coal mines and cut lots of jobs.
And in 1990, there was fighting on the streets of London because of a new tax - called the poll tax - that people thought hit poor people harder than the rich.
Margaret Thatcher was thought to have survived on just four hours sleep a night - and got the nickname "the Iron Lady" for being tough and sticking to her decisions. In 2012 a big movie was made of her life, called The Iron Lady.
After 11 years in power - and winning three elections - she'd been prime minister for longer than anyone else in the 20th century.
But eventually even her own party thought she had been in charge too long - and she left Downing Street in tears.
Now, more than 20 years on, she is recognised as a prime minister who transformed the face of Britain - and will be remembered as one of the most important figures in modern British history.