Princess Margaret, Countess of Snowdon (Margaret Rose; 21 August 1930 – 9 February 2002), was the younger daughter of King George VI and Queen Elizabeth The Queen Mother. She was the younger sister and only sibling of Queen Elizabeth II.

Margaret was born when her parents were the Duke and Duchess of York, and she spent much of her childhood with them and her elder sister. Her life changed at the age of six, when her father succeeded to the British throne following the abdication of his brother Edward VIII. Margaret's sister became heir presumptive, with Margaret second in line to the throne. Her position in the line of succession diminished over the following decades as Elizabeth's children and grandchildren were born. During the Second World War, the two sisters stayed at Windsor Castle despite suggestions to evacuate them to Canada. During the war years, Margaret was too young to perform official duties and continued her education, being nine years old when the war broke out and turning fifteen just after hostilities ended.

From the 1950s onwards, Margaret became one of the world's most celebrated socialites, famed for her glamorous lifestyle and reputed romances. Most famously, she fell in love in the early 1950s with Peter Townsend, a married RAF officer in the royal household. In 1952, her father died, her sister became queen, and Townsend divorced his wife. He proposed to Margaret early in the following year. Many in the government believed that he would be an unsuitable husband for the Queen's 22-year-old sister, and the Archbishop of Canterbury refused to countenance her marriage to a divorced man. Margaret abandoned her plans with Townsend. In 1960, she married Antony Armstrong-Jones, whom Elizabeth created Earl of Snowdon. The couple had two children, David and Sarah. Margaret's marriage to Lord Snowdon became strained, with both of them engaging in extramarital affairs. They separated in 1976 and divorced in 1978. Margaret did not remarry.

Margaret was a controversial member of the British royal family. Her divorce received much negative publicity, and her private life was for many years the subject of speculation by media and royal correspondents. Her health deteriorated in the last twenty years of her life. She was a heavy smoker for most of her adult life, and had a lung operation in 1985 and a bout of pneumonia in 1993, as well as three strokes between 1998 and 2001. Margaret died in 2002 aged 71, after suffering her fourth stroke.

Margaret was born at 9:22 pm on 21 August 1930 at Glamis Castle in Scotland, her mother's ancestral home, and was affectionately known within the royal family as Margot. She was the first member of the royal family in direct line of succession to be born in Scotland since the 1600s. She was delivered by Sir Henry Simson, the royal obstetrician. The Home Secretary, J. R. Clynes, was present to verify the birth. The registration of her birth was delayed for several days to avoid her being numbered thirteen in the parish register. Margaret was baptised in the private chapel at Buckingham Palace on 30 October 1930 by Cosmo Lang, the Archbishop of Canterbury.

At the time of her birth, Margaret was fourth in the line of succession to the British throne. She was the younger daughter of Prince Albert, Duke of York (later King George VI), and Elizabeth, Duchess of York (later Queen Elizabeth The Queen Mother). Her father was the second son of King George V and Queen Mary, and her mother was the youngest daughter of Claude Bowes-Lyon, 14th Earl of Strathmore and Kinghorne, and Cecilia Bowes-Lyon, Countess of Strathmore and Kinghorne. The Duchess of York originally wanted to name her second daughter Ann Margaret, as she explained to Queen Mary in a letter: "I am very anxious to call her Ann Margaret, as I think Ann of York sounds pretty, & Elizabeth and Ann go so well together." George V disliked the name Ann but approved of the alternative, Margaret Rose.

Margaret's early life was spent primarily at the Yorks' residences at 145 Piccadilly (their town house in London) and Royal Lodge in Windsor. The Yorks were perceived by the public as an ideal family, but unfounded rumours that Margaret was deaf and mute were not completely dispelled until her first main public appearance at her uncle Prince George's wedding in 1934.

Margaret was educated alongside her sister, Elizabeth, by their Scottish governess, Marion Crawford. Margaret's education was mainly supervised by her mother, who in the words of Randolph Churchill "never aimed at bringing her daughters up to be more than nicely behaved young ladies". When Queen Mary insisted upon the importance of education, the Duchess of York commented, "I don't know what she meant. After all I and my sisters only had governesses and we all married well — one of us very well". Margaret was resentful about her limited education, especially in later years, and aimed criticism at her mother. However, Margaret's mother told a friend that she "regretted" that her daughters did not go to school like other children, and the employment of a governess rather than sending the girls to school may have been done only at the insistence of their grandfather George V. J. M. Barrie, author of Peter Pan, read stories to the sisters as children.