Transitional epithelium contains cells that are flattened and cells that are cuboidal; hence the name "transitional".  You can find transitional epithelium in the bladder (shown below) and in the first expansion of the ureters as they leave the kidneys (called a calyx) (shown in the above photograph). 

Another way to remember transitional epithelium is that the cells have the capacity to stretch and flatten so the calyx and bladder can expand.  What structures allow this distensibility?

Note that in the BLADDER transitional epithelium (shown below) the cells nearest the apex are rounded.  This is a diagnostic feature of this epithelium.