Weeds have been controlled by humans since the beginning of agriculture by means of mechanical tools or by hand. It was early in the 20th century that some inorganic compounds were Rrst used with this aim. The discovery of the herbicidal properties of 2,4-D (2,4-dichlorophenoxyacetic acid) in 1945 can be considered the initiation of use of organic herbicides in agriculture. Since then, more than 130 different active compounds have been synthesized for their application as herbicides. These compounds can be grouped, according to their chemical structures, into different herbicide classes (Table 1). Compounds belonging to the principal herbicide groups will be considered in this study. These compounds control weeds in a variety of ways, showing different modes of action, selectivity and application characteristics. Soil-applied herbicides are absorbed by roots or emerging shoots and foliageapplied herbicides are absorbed into the leaves, where they may be translocated to other parts of the plant. The active ingredient of a herbicide is a compound, usually obtained by synthesis, which is formulated by a manufacturer in soil particles or liquid concentrates. These commercial formulations of herbicides are diluted with water before application in agriculture at the recommended doses. Herbicide formulations generally contain other materials to improve the efRciency of application. Analysis of herbicide formulations was initially carried out by wet chemical procedures, such as determination of total chlorine, nitrogen or phosphorus, or by spectrometric procedures like ultraviolet absorption. The development of gas chromatography (GC) allowed the analysis of these compounds in commercial formulations with high selectivity and sensitivity. The analytical procedure is commonly based on the dissolution of a known amount of the formulation in an organic solvent, which often contains an internal standard to improve the precision and accuracy of the determination. An aliquot of this solution is analysed by GC. Packed columns were used initially, but have now been replaced by capillary columns of low or medium polarity and Same ionization is the detection technique more widely used.When herbicides are not volatile or thermally stable, high performance liquid chromatography (HPLC) is the preferred technique for their determination in commercial formulations. Figure 1 shows the gas chromatographic separation of a mixture of phenoxy esters.