Cross processing is a term which describes the act of processing photographic film in a solution that was not intended for the film. It is a process which is mostly (though not exclusively) done with colour film. You could either process colour positive film (slides) in C-41 chemicals, which creates a negative image on a base that is colourless or you can develop a colour negative film with E-6 chemiclas which will leave you with the positive image however it will be on the orange based film, which you usually use for colour negative film.
This process can get you very unpredictable and unnatural colours and extreme contrast. For this reason I have not used it many times. It can look great but since it is so unpredictable I wasn't impressed with it.
The photo I have chosen for today was taken in 2010 in London, and I used cross processing to develop it. The colours are really crazy and with that image it works, but with some of the others I think I lost a little of the atmosphere that I was out to capture.