Watermelon/melon is a healthful and popular fruit in the world. The main indices of internal qualities of watermelons/melons are soluble solids content (SSC), firmness, internal defect, and maturity. Internal qualities of watermelons/melons were mostly assessed by human experience in the last decades. It was inconsistent, subjective, and slow. Research workers have paid attention to this issue and have done a lot of work on measuring the internal qualities of watermelons/melons nondestructively and rapidly. This review gives a comprehensive overview of the nondestructive determination techniques on internal quality assessment of watermelons/melons including acoustic technology, dynamic technology, electrical and magnetic technology, X-ray and computed tomography, and near infrared (NIR) spectroscopy. A comparison of these techniques indicates that on-line measurement by Vis/NIR spectroscopy will play an important role in the further study. (C) 2010 Elsevier Ltd. All rights reserved.