Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that regulates the expression of genes related to cell cycle, cell survival, and immune response associated with cancer progression and malignancy in a number of cancer types. Once activated, STAT3 forms a homodimer and translocates to the nucleus where it binds DNA promoting the translation of target genes associated with antiapoptosis, angiogenesis, and invasion/migration. In normal cells, levels of activated STAT3 remain transient; however, STAT3 remains constitutively active in approximately 70% of human solid tumors. The pivotal role of STAT3 in tumor progression has promoted a campaign in drug discovery to identify small molecules that disrupt the function of STAT3. A range of approaches have been used to identify novel small molecule inhibitors of STAT3, including high-throughput screening of chemical libraries, computational-based virtual screening, and fragment-based design strategies. The most common approaches in targeting STAT3 activity are either via the inhibition of tyrosine kinases capable of phosphorylating and thereby activating STAT3 or by preventing the formation of functional STAT3 dimers through disruption of the SH2 domains. However, the targeting of the STAT3 DNA-binding domain and disruption of binding of STAT3 to its DNA promoter have not been thoroughly examined, mainly due to the lack of adequate assay systems. This review summarizes the development of STAT3 inhibitors organized by the approach used to inhibit STAT3, the current inhibitors of each class, and the assay systems used to evaluate STAT3 inhibition and offers an insight into future approaches for small molecule STAT3 inhibitor development.