I spent the week investigating the process of 'pushing' film. In simple terms it is the practice of setting the ISO on your camera higher than the box speed, effectively underexposing the film. In order to ensure usable photographs you then  over developing the film during developing process. There are pros and cons to this practice - you can take pictures in far darker environments or at faster speeds, but you lose shadow detail and gain contrast or grain. My feeling is that the cons don't really effect my style of photography, so I pushed a roll of Kodak T-Max 400 to 1600, added some extra dev time and.......well, the results speak for themselves.