Renal cysts are fluid-filled structures in the kidney that are not continuous with the nephron or collecting system, believed to originate from diverticuli of distal convoluted tubule or collecting tubules, possibly due to weakening of basement membrane. Cystic renal lesions are ubiquitous and seen in a wide spectrum of sporadic,inherited and systemic diseases. Ra­diologists must be aware of the imaging appearances of cystic lesions, understand their pathophysiology to make a diagnosis and provide optimal recommenda­tions regards management and follow-up imaging.The Incidence of sporadic cysts increases with age: 0.2% from 0-18 yrs,20% from 20-40 yrs,33% from 41-60 yrs. Most simple cysts grow slowly with time:3.9 mm per year for <50 yrs old,1.8 mm per year for >50 yrs old. Some may involute and disappear over time. Risk factors include : Increasing age, ESRD on haemodialysis,Polycystic kidney disease- both autosomal dominant and recessive types,Von Hippel-Lindau syndrome and tuberous sclerosis. Occasionally detection of incidental cystic renal disease may result in diagnosis of inherited cystic diseases if unrecognized previously. The vast majority are benign simple cysts, but complex cystic renal lesions are not uncommon which can have a wide differential diagnosis. The Bosniak classification system provides a useful tool for categorizing, standardizing reporting and managing renal cysts, According to the current classification, lesions in category I correspond to simple cysts without septa or vegetations, with thin and smooth walls, and no contrast enhancement after the administration of intravenous contrast agents .Category II includes cysts with thin septations, minimally thick walls and fine parietal calcifications, and no contrast enhancement after intravenous contrast agent injection. Homogeneous hyperdense cysts ≤ 3.0 cm are included in this category. Lesions with irregular and/or thick septa, with course calcifications, and clear enhancement after intravenous contrast injection are described as category III . Category IV is reserved for lesions with septa or walls with well-defined solid components that demonstrate contrast-enhancement after intravenous contrast injection.. Category I &II are considered benign and managed conservatively. Category II-F corresponds to indeterminate lesions, which, although not sufficient to indicate surgical exploration, suggest a slight risk of malignancy. Category III & IV suggest a malignancy/ neoplasm and are usually managed surgically. Cystic renal neoplasms include a heterogeneous of tumors such as cystic RCC, multilocular cystic nephroma and more recently recognized mixed epithelial stromal tumor (MEST). They can often be difficult to differentiate on imaging as they have common morphological features but have differing implications for follow-up after resection. Uncommonly renal infections such as pyogenic abscess or hydatid disease(Echinococcal disease) may present as complex renal cystic lesions mimicking tumors and must be borne in mind . MRI has been widely used in the characterization of cystic lesions in kidneys and other organs, usually with better performance than CT. MRI better demonstrates the presence of thin septa in cystic lesions, in particular within cysts < 2.0 cm). The enhancement of thin septa, described as capillary or hair-like enhancement, is much more conspicuous at MRI than at CT, providing greater confidence in their detection and for denying the absence of contrast-enhancement. Other advantage of MRI is the identification of contrast-enhancement of internal septa within hemorrhagic cysts utilizing subtraction techniques. Diffusion-weighted MRI has been suggested as an adjunct to routine MR imaging and allows for indirect evaluation of tumor cellularity. In complex cystic lesions, restricted diffusion in solid components was shown to have a high positive predictive value for cancer Guidelines for surveillance : Category I and II renal cysts, do not require further imaging or follow-up. Patients in Category IIF, because of the approximate 5% malignant risk, do require periodic imaging. (There is no consensus or evidence based interval determined for follow-up imaging.) Combination of ultrasound and MRI should be considered as follow-up for Bosniak IIF and reduces the lifetime radiation dose (once the lesion has been characterized by triphasic CT scan) in patients younger than 50 years.One suggestion surveillance theme has been 6monthly imaging for at least 5 years. For Category III (50% malignant risk) and category IV (75% to 90% malignant risk), surgical excision is recommended.