An education expert has caused controversy by questioning the scientific validity of the term dyslexia.
Dyslexics might have trouble with reading, writing or numbers
Professor Julian Elliott, from Durham University, said the term was an "emotional construct", not a scientific one and that experts could not agree on a definition of dyslexia or on how to treat it.
The BBC News website looks at background to the condition, who has it and the way it is treated.
Q: What is dyslexia?
Dyslexia is commonly understood to be a term covering a range of types of learning difficulty where someone of normal intelligence has persistent and significant problems with reading, writing, spelling. Some experts say the problems can also be in maths or musical notation.
It is understood to be a congenital and developmental condition affecting the brain. Scientists in recent years have said they have identified genes common to families with dyslexia.
Q: What are the symptoms?
Dyslexic symptoms include poor reading, spelling and handwriting but can also cover wider problems with concentration and co-ordination.
The British Dyslexia Association says the condition can also involve difficulties with numbers or reading musical notes.
Who is affected?
Up to six million Britons are believed to have dyslexia. 4% of the population is severely dyslexic and a further 6% have limited problems.
Which famous people have had dyslexia?
Sir Winston Churchill, Albert Einstein, Sir Richard Branson and Princess Beatrice. The princess, 17, recently spoke about her condition, saying she had been struggling with books that her younger sister, Eugenie, could read.
What is the current controversy about?
Psychology professor Julian Elliott has angered dyslexia organisations by saying dyslexia is an "emotional construct", not a "scientific function", arguing that he would have little confidence in any diagnosis of dyslexia because there is no consensus about how to define it.
He condemned what he called the "dyslexia industry" for its "spurious" link between diagnosis and treatment.
But Professor Susan Tresman, of the British Dyslexia Association, insists there is a scientific consensus on what constitutes dyslexia and says educational psychologists and specialist teachers can accurately diagnose the condition.
The government recognises dyslexia as a learning difficulty.
How is it treated?
A range of treatments have been developed, from reading schemes to the use of lights and sounds and exercises to engage different parts of the brain.
Do dyslexics receive extra help in school?
That depends on the school and also on the severity of the problem. If a child has a statement of special needs because they are severely dyslexic, they would be entitled to extra support.
In national exams, pupils with a formal diagnosis of dyslexia can receive up to 25% extra time to complete their work.