Histone lysine methyltransferase 2 (KMT2) proteins form multimeric enzymatic complexes that methylate lysine 4 on histone H3 (H3K4) at transcription regulatory elements in the genome. A strong association of H3K4 methylation with active transcription has led to intense efforts to reveal the functional involvement of KMT2 complexes in transcriptional regulation. A number of biochemical and cellular studies have shown that KMT2 complexes regulate transcription of target genes via H3K4 methylation. However, in many cases, loss of KMT2 complex enzymatic activity fails to fully account for observed transcriptional defects. Accumulating evidence indicates that, in certain contexts, KMT2 complex-mediated transcriptional regulation can occur in an H3K4 methylation-independent manner. Here, we comprehensively review functions of KMT2 complexes in gene expression, focusing on what we currently know about the molecular mechanisms by which the KMT2 complexes regulate transcription. We also discuss how aberrant transcriptional regulation by KMT2 complexes contributes to different human diseases, such as cancer.