Do you need to know what you're writing before you start? Or can you wait till it's time to send it out? And do you want to write to a strict genre anyway or are you more of a mould-breaker?From the point of view of a publishing house and a bookseller, the clearer you can be about your genre, the better.
Let's start with the basics - are you writing fiction or non-fiction? This is not as stupid a question as you might think. I have read a number of submission letters that start with 'My novel is all TRUE', or 'I'm writing about an important issue but I've turned it into fiction'. Try and draw a line, and decide which side of it you're writing.
Then - adult or children? Very very few books are genuinely cross-over titles. I know Philip Pullman and JK Rowling managed it - but what they actually did was to write children's books that appealed to adults, rather than attempt a hybrid.
If you're writing for children, what age are you aiming it at? The most common mistake I see when reviewing submissions is that the author has aimed a book at 'children from 3 to 13'. Very few - if any- books will appeal across that wide an age bracket. Have a look at your local bookshop and how they divide up books for children, and use that as your blueprint for where your book would sit.
All that seems strikingly easy compared to catergorising adult fiction. Is your novel chick-lit (or has that had its day - see this Guardian article), crime, romance, historical, literary, mass-market - or something else? And how can you tell?
By the time you're seriously writing a book, you should know what sort of book you're writing. Think of it less as pigeon-holing, and more as defining the sort of reader you're looking for.
Remember, you don't have to restrict yourself to one area of writing. Jo Nesbo, dark Norwegian crime writer, also writes humorous children's books. Louisa May Alcott wrote inspiring children's fiction, and shocking 'sensation' novels. Iain Banks writes literary fiction, while his alter-ego Iain M.Banks writes science fiction. Just make sure each piece of writing contains its own coherent identity.
What genre are you writing currently? And how easy is it for you to define your genre? An inspiring blogger is writing 12 books in 12 different genres over a year - see Ali George's website. Why not try a different genre, just as an exercise?
Cressida Downing
(Editorial Consultant)

Jonathan Hopkins on October 4, 2011
Think I'll try to get good at one, first.
Oliver Gunne on October 5, 2011
Genre is an accepted style or enduring category of literature - something an audience has accepted before and is familiar with its structure and so is more likely to buy. However, if you are truly looking for something unique (though not necessarily original) then you should consider why we tell stories? Doctor Samuel Johnson "... to show how small a quantity of real fiction there is in the world; and that the same images, with very little variation, have served all authors who have ever written." Christopher Booker boiled it all down to 'The Seven Basic Plots' albeit in several hundred pages (and not one wasted). It's what makes people sit down and listen / read / watch a story unfold. I am often loathe to offer an opinion because it's always met with the subjective unconscious mind that programs our emotional responses. Or better put by Alexander von Humboldt "First people deny a thing; then they belittle it; then they say it was known all along."
Patrick Fox on October 8, 2011
I don't know anything about genre, I just write.
Neil Ansell on October 8, 2011
There may be hidden advantages to crossing genres - my book is currently on three different shelves, on three different floors, of the local Waterstones. Got to be good for visibility, and for stock levels.
john wilson on October 10, 2011
My first book has a cast of talking birds. The themes covered and the very human issues they deal with I have aimed towards Young Adult. As for genre, I have no idea. Not sure what every genre actually means, and the list seems to be growing Children's Literature, Adult Literature, Teen Books, Fantasy, Fiction, Literary Fiction, Thrillers, True Crime, Crime Fiction, Science Fiction, Satire... I was hoping an agent might help categorise it with me, although, despite a coop full of chickens, will vigorously resist 'chick-lit'.