Technical Abstract: Calcium oxalate crystal formation in plants appears to play a central role in a variety of important functions, including tissue calcium regulation, protection from herbivory, and metal detoxification. Evidence is mounting to support ascorbic acid as the primary precursor to oxalate biosynthesis. The ascorbic acid utilized in oxalate biosynthesis is synthesized directly within the calcium oxalate crystal-accumulating cell, called the crystal idioblast. Several unique features of the crystal idioblast have been proposed as factors that influence calcium oxalate formation. These features include an abundance of endoplasmic reticulum (ER), acidic proteins, cytoskeletal components, and the intravacuolar matrix. A number of mutants defective in different aspects of calcium oxalate crystal formation have been isolated. Cellular and biochemical characterizations of the various mutants have revealed mutations affecting crystal nucleation, morphology, distribution, and/or amount. Such mutants will be useful tools in continued efforts to decipher the pathways of crystal formation and function in plants.