The Bosniak classification system of renal cystic masses divides renal cystic masses into five categories based on imaging characteristics on contrast-enhanced CT. It is helpful in predicting a risk of malignancy and suggesting either follow up or treatment. Classification: Bosniak I -Simple cyst , imperceptible wall; Work up:nil; Percentage malignant: ~0% Bosniak 2 - Minimally complex, a few thin <1 mm septa or thin calcifications (thickness not measurable); non-enhancing high-attenuation (due to proteinaceous or haemorrhagic contents) renal lesions of less than 3 cm are also included in this category; these lesions are generally well marginated; Work-up: nil; Percentage malignant: ~0% Bosniak 2F - Minimally complex, increased number of septa, minimally thickened with nodular or thick calcifications there may be perceived (but not measurable) enhancement of a hairline-thin smooth septa hyperdense cyst >3 cm diameter, mostly intrarenal (less than 25% of wall visible); no enhancement; Requiring follow-up: needs ultrasound/CT follow up - no strict rules on the time frame but reasonable at 6 months; Percentage malignant: ~ 5% Bosniak 3 - Indeterminate thick, nodular multiple septa or wall with measurable enhancement, hyperdense on CT; Work-up: paial nephrectomy or radiofrequency ablation in elderly or poor surgical candidates; Percentage malignant: ~55% Bosniak 4 - Clearly malignant solid mass with a large cystic or a necrotic component; Treatment: paial or total nephrectomy; Percentage malignant: ~100%.