Huntington's disease (HD) is associated with increased expression levels and activity of tissue transglutaminase (TG2), an enzyme primarily known for its cross-linking of proteins. To validate TG2 as a therapeutic target for HD in transgenic models and for eventual clinical development, a selective and brain-permeable inhibitor is required. Here, a comprehensive profiling platform of biochemical and cellular assays is presented which has been established to evaluate the potency, cellular efficacy, subtype selectivity and the mechanism-of-action of known and novel TG2 inhibitors. Several classes of inhibitors have been characterized including: the commonly used pseudo-substrate inhibitors, cystamine and putrescine (which are generally nonspecific for TG2 and therefore not practical for drug development), the various peptidic inhibitors that target the active site cysteine residue (which display excellent selectivity but in general have poor cellular activity), and the allosteric reversible small-molecule hydrazides (which show poor selectivity and a lack of cellular activity and could not be improved despite considerable medicinal chemistry efforts). In addition, a set of inhibitors identified from a collection of pharmacologically active compounds was found to be unselective for TG2. Moreover, inhibition at the guanosine triphosphate binding site has been examined, but apart from guanine nucleotides, no such inhibitors have been identified. In addition, the promising pharmacological profile of a TG2 inhibitor is presented which is currently in lead optimization to be developed as a tool compound.