Helical CT is the most useful imaging modality to evaluate kidney diseases. Different imaging protocols are used to assess the correct diagnosis in each clinical situation. The nephrographic phase (between 90 and 100 s of delay after injection) is more accurate than the cortical phase (between 30 and 40 s of delay) to depict and characterise small renal masses. Multiplanar and 3D reconstruction are useful to plan partial kidney surgery or percutaneous surgery of lithiasis. In emergency, spiral CT, if available, is suitable to assess renal colic. Spiral CT is the best modality to evaluate traumatic kidney.