Just wanted to ask a few questions about cadenzas and those by Haydn in particular. If they are notated in the music would they have been composed by Haydn or added by the publisher or someone else?

Looking on Youtube I've noticed that people playing the same pieces play different cadenzas. Is this because they have written their own (some are much simpler) or are using a different edition which contains different cadenzas?

And is it always expected that they are included or are cadenzas optional especially if there a several in one piece if you have time constraints?

Cadenzas are sometimes written by the composer , and sometimes left to the performers imagination. It is an opportunity to 'show off' a bit.
Sometimes a later composer will write a cadenza for an earlier composers work. Indeed there may have been several written for any given work.
In a performance a marked cadenza, (whether by the composer, or otherwise) should be played, it does enhance the performance, if theres not one printed, improvise, and usually end with a ubiquitous trill to lead into the tutti.

More here :- http://en.wikipedia.org/wiki/Cadenza

Thanks Fsharpminor. I had read the Wiki article but my main question I suppose is why other people are playing different unimprovised cadenzas to the ones notated in our books......

If the ones notated in the book aren't by the original composer, i.e. they are editorial realisations, then people may just fancy doing their own show-off-cadenza, rather than playing one that's already written down. After all, my cadenza is every bit as close to the composer's intent as the editor's one!

Even if it is one written by the composer, I imagine that some people make up their own anyway - they may not realise it's the composer's, or they may prefer to do their own thing (it's not often you get the chance to, usually you have to follow what's written to some extent!)

Plus some of the cadenzas in books are poor quality - not mentioning any particular edition of Mozart violin concertos

Composers tended not to write cadenzas for pieces in the earlier, classical period and expected the performer to 'improvise' their own (not usually improvised on the spot) using themes and material from the piece. Those composers that performed their own works (eg Mozart) would write their own cadenzas but these were not often published with the music.

Later sometime from the Romantic period composers began to write the cadenzas they expected the soloist to perform. I know more about violin repertoire here, so talking about violin concerti now - for eg Mendelssohn and Tchaikovsky the cadenzas were written by the composer. Brahms it wasn't though Joachim's (who advised Brahms) is very often performed. Beethoven has several cadenzas that are often performed.

These days performers tend to play one of the cadenzas written by their forbears, such as Kreisler, Heifetz, Joachim etc, and it is possible to buy published cadenzas for concerti, very rarely these days do they write their own. Vengerov plays his own for Brahms, Beethoven and Mozart but he is rather special .

For something very different from the ordinary, the composer Schnittke wrote fascinating cadenzas for the violin concerto: http://www.youtube.com/watch?v=yGRKUl4TrJU

As noted, player-composers such as Mozart would improvise at the keyboard, so their cadenzas were not necessarily written down. Often the cadenza used by the first performer would get written down. Certainly until well into the C19th it was more usual for a soloist to write their own to show-off.

As the C19th progressed (probaly as the idea of the 'star' soloist grew?) it was more common to use a cadenza written by a 'big name' or by the composer rather than the individual soloist.

The C20th saw a revival of interest in soloists writing their own cadenzas.

True, but it also saw the expansion of fully-composed cadenzas, to the point where in pieces such as Shostakovich's first violin concerto the cadenza is almost a movement in its own right.

And then there's Schnittke's 4th Violin concerto cadenza

Even I could play that one. In fact I think I'll play it for every concerto

Ooh just have to say I love that concerto !

The last movement of his first string quartet is titled 'Cadenza' - a cadenza for the whole quartet, as the conclusion of the piece. Typical Schnittke, playing around with expected structures and concepts.