Allelopathy is one of the oldest examples of environmental interactions between higher plants and other living organisms. Biologists, chemists, microbiologists, ecologists, molecular genetists and other scientists work together in this scientific discipline. Assessment of the allelopathic effects is complicated as it depends on the donor and acceptor organisms, and involves not only the toxicity of individual allelochemicals, but also their interactive effects, including antagonistic and synergistic effects. The present European Union environmental policy requests to apply procedures and practices that minimize the toxic environmental effects of the allelochemicals. The present book provides a concise introduction to the walnut allelopathy in the field and laboratory allelopathic tests and molecular background of this phenomenon. Short characteristics of Juglandaceae, and effect of walnut and juglone on other organisms including human are included too. Students and researchers new to the subject may find this text an invaluable guide.