Technical Abstract: Native and cultivated plants exist in soil environments that can contain phytotoxic levels of a number of different metals, including Al, Pb, Cd, Zn, Hg, Cu, Cr, Fe, Mn and Ni. In recent years, a growing awareness by plant biologists of the importance of environmental issues in agriculture has been reflected in the interest expressed in plant responses to metal toxicities in the environment. Since the last review in this series on plant metal toxicities in 1978, a very significant portion of the research in this area has focused on the mechanisms of aluminum (Al) phytotoxicity and genetically-based Al resistance. The intense research effort currently underway on Al toxicity is indicative of the agronomic importance of this problem, in that Al toxicity is the primary factor limiting crop productivity on acid soils which comprise large areas of the world's lands, particularly in the tropics and subtropics. Thus, it is an important factor rlimiting food production in many developing countries. In order to better develop Al-resistant crops via plant breeding and biotechnology, a considerable research effort is also underway to understand the fundamental processes that confer Al resistance in plants. Despite the research that has been conducted on the cellular and physiological bases of Al toxicity and resistance, it is a research area that is fraught with confusion and controversies. Therefore, this review will address our current state of understanding of these topics, with emphasis on the areas of controversy.