The present Key Event based Test Guideline (TG) addresses the human health hazard endpoint skin sensitisation, following exposure to a test chemical. Skin sensitisation refers to an allergic response following skin contact with the tested chemical, as defined by the United Nations Globally Harmonized System of Classification and Labelling of Chemicals (UN GHS). This TG is proposed to address the activation of dendritic cells, which is one Key Event on the Adverse Outcome Pathway (AOP) for Skin Sensitisation. It provides four in vitro test methods addressing the same Key Event on the AOP: (i) the human cell Line Activation Test or h-CLAT method, (ii) the U937 Cell Line Activation Test or U-SENS, (iii) the Interleukin-8 Reporter Gene Assay or IL-8 Luc assay and (iv) the Genomic Allergen Rapid Detection for assessment of skin sensitisers (GARD™skin). All of them are used for supporting the discrimination between skin sensitisers and non-sensitisers in accordance with the UN GHS. The test methods described in this TG either quantify the change in the expression of cell surface marker(s) CD54 and CD86, the cytokine IL-8, or a series of genes (genomic biomarker signature) that are associated with the process of activation of monocytes and DC following exposure to sensitisers.