The goal of this observational, prospective, multi-national clinical study is to assess overall survival of patients who are diagnosed with incidental, histologically (biopsy) confirmed, \<4 cm Renal Cell Carcinoma (RCC) and are managed conservatively with active surveillance. The primary endpoint is overall survival. The Secondary endpoints are tumor growth rate, progression rate, cancer-specific survival, progression-free survival, identification of clinical and pathological variables and molecular and genetic markers that correlate with growth rate and progression. The main question it aims to answer is: patients with RCC (less than 4 cm) diagnosis can be managed with active surveillance instead treated with invasive curative procedure? For all participants a percutaneous biopsy of the renal mass will be arranged in all cases to histologically confirm the diagnosis of RCC (unless a diagnostic biopsy has been acquired in the previous 6 months). As a minimum, two samples will be used for diagnostic purposes while remaining core(s) will be preserved for molecular studies. Then, all patients will be under active surveillance, which is defined as the initial monitoring of tumor size by serial abdominal imaging (US, CT, or MRI) Follow-up visits will be scheduled 3 (optional) and 6 months after diagnosis, every 6 months up to 3 years and yearly thereafter. A follow-up visit will also be carried out at the time of progression when it occurs. Follow-up visits will include medical history and physical examination (optional), and assessment of concurrent medications, blood and urine collection and storage if participating in translational activities, cross-sectional abdominal and chest imaging exams. Follow-up percutaneous biopsies of the renal tumor are not mandatory, but can be performed when considered clinically important.