Cadaveric or living donor renal transplantation is commonly performed in individuals with end-stage renal disease. In recent years, gray-scale sonography, coupled with color Doppler sonography (CDUS), power Doppler sonography (PDUS), or spectral Doppler sonography, has become the primary imaging modality for these patients. Postoperative serial sonography is performed to detect complications and aid in posttransplant management. In addition, sonography is used to guide percutaneous aspiration of fluid or biopsy to diagnose rejection or renal and perirenal masses. In this article we discuss the spectrum of sonographic findings, both vascular and nonvascular, of renal transplant complications, including but not limited to renal arterial and venous stenosis and thrombosis, peritransplant collections (lymphoceles, hematomas, urinomas, and seromas), posttransplant lymphoproliferative disorder, and postbiopsy complications (hematomas, pseudoaneurysms, and arteriovenous fistulas). We correlate sonographic findings with those from other imaging modalities (such as angiography, CT, and MRI) and findings at surgery and pathology when possible.