The present invention relates to immunoglobulin single variable domain sequences that are directed against (as defined herein) OX40L, as well as to compounds or constructs, and in particular proteins and polypeptides, that comprise or essentially consist of one or more such immunoglobulin single variable domain sequences. In particular these immunoglobulin single variable domain sequences can block binding of OX40L to OX40.The immunoglobulin single variable domains, compounds and constructs can be used for prophylactic, therapeutic or diagnostic purposes, such as for the treatment of inflammatory disease and/or disorder such as e.g. asthma, allergic asthma, chronic colitis, Crohn's disease, inflammatory bowel disease, and/or arthrosclerosis.