For the objective comparison of small structures several indicators and descriptors have been devised which aim to express various characteristics of compounds such as physicochemical properties, shape, topology and total molecular complexity. Such indicators may be invaluable in synthesis planning and analysis of compound libraries. Complexity descriptors attempt to translate intuitive qualitative features describing this property into quantitative measures, and both individual intuitive features and complexity descriptors have their own bias. Complexity metrics that not only match chemical intuition but also correlate to biological outcomes, such as target selectivity, are of particular value to the drug discovery community.