The latest previous comprehensive review of allelopathy (Rice, 1974) covered research done primarily prior to 1973. There have been numerous reviews published in recent years on specific phases of allelopathy (see III), but most of them covered research results published primarily through 1972 also. This review has been restricted, therefore, largely to research conducted subsequent to 1972, except for background information and significant papers overlooked in previous reviews. There has been almost an exponential increase in the rate of publication of papers concerned with allelopathy. Research has been particularly active in relation to the roles of allelopathy in agriculture, forestry, phytopathology, patterning of vegetation, algal succession, and old-field succession. Our increasing knowledge of the conditions under which certain crop residues cause allelopathic effects to subsequent crops should enable us soon to guard against such effects. We are on the threshold of breeding crop plants that will inhibit the chief weeds in a given area through allelopathic action, and thus decrease the need for synthetic weed killers. Our understanding of allelopathic interactions of various plant species has already been used advantageously in reforestation, and future developments are very encouraging. Our increasing knowledge of allelopathy is aiding greatly in our understanding of many ecological phenomena, such as succession and patterning of vegetation. Evidence is mounting that inhibition of nitrification increases as succession progresses toward the climax vegetation, at least in many vegetation types. This leads to a decrease in the loss of nitrogen. Addition of inhibitors to arable lands to prevent nitrification has proved to be valuable in preventing loss of nitrogen and in increasing crop yields. There has been a rapid advance in our knowledge of mechanisms of action of known allelopathic compounds, at increasingly more fundamental levels. We have known for several years, for example, that some of these compounds inhibit uptake of various minerals, but have not known the mechanism of the action. Recent research indicates this may be due, at least in part, to inhibition of a plasma membrane ATPase that is involved with ion absorption. A thorough knowledge of mechanisms of action of allelochemics is essential to our overall understanding and appreciation of the field of allelopathy.