Many analytical problems require more resolution than a single chromatographic technique can provide. In such cases the separation power can be enhanced by using more than one separation technique or mechanism. The sample is then dispersed in different time dimensions. The resolution which can be obtained depends strongly on the differences between these dimensions. The highest resolution is gained when there is no correlation between the separations, the dimensions being orthogonal to each other. This orthogonality can be obtained by hyphenating pairs of techniques together, as with liquid chromatography (LC) and gas chromatography (GC), but also by coupling techniques which are based on different separation mechanisms, such as size-exclusion LC and reversed-phase LC. A multidimensional technique is comprehensive when the whole sample is dispersed in more dimensions, rather than only a single fraction, as in heart-cut techniques. This review focuses on the advantages and possibilities of comprehensive multidimensional (MD)GC, but the theory is also applicable to other hyphenated multidimensional separation techniques.