You have to know what kind of transformer it. magnetic or electronic. the dimmer must specify if it is for electronic transforer or magnetic. you also have o realize that some dimmers require a neutral connection to the dimmer otherwise it will not work.
Some low voltage transformers can be dimmed with regular dimmers. WAC makes them for their under-cabinet puck lights. Many will not work with standard dimmers though, so read any literature that comes with the transformer. Like Edward said, you need to know if it is a magnetic or electronic transformer.
A possible (if costly) option for dimming LV lighting with a conventional magnetic transformer is to run the transformer primary from a variac (variable autotransformer).