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Cystic renal disease is very common and the detection of complex cystic masses in the kidney has increased dramatically over the last few decades with increased cross-sectional imaging. Cystic renal cell carcinoma represents 5–7% of all renal tumours. The ability to differentiate between those who are benign from those malignant represents a major challenge. The controversy and difficulty lies in those complex cystic renal masses that lie between Bosniak II and III. This review focuses on the diagnosis and management of complex renal cysts, examining the evidence for biopsy and looks at novel diagnostic techniques.There are varying results in interpretation of complex cysts and this is heavily dependent on interobserver variability. Bosniak classification is a valuable tool for the clinician. Computed tomography remains the gold standard, however, newer diagnostic imaging modalities such as contrast-enhanced ultrasonography is demonstrating promising results. Percutaneous biopsy of these complex lesions would appear to be well tolerated and adequate for diagnosis.The ability to accurately identify complex renal cysts that are malignant remains a major challenge to the clinician. Regularly there is no other way to identify malignant features than to biopsy or continue surveillance. Recent literature would suggest that percutaneous biopsy has a good yield and is accurate in the majority of cases. Current apprehension with regard to iatrogenic complications from percutaneous renal biopsy may be exaggerated, as complication rates are higher in patients with intrinsic real disease rather than this cohort of patients. The overlap in diagnosing Bosniak IIf versus III is heavily influenced by interobserver variability and this needs to be a significant factor in making any decision.