Recent insights into the molecular pathogenesis of colorectal cancer (CRC) have given rise to specific target-directed therapies, including monoclonal antibodies (mAb) against epidermal growth factor receptor (EGFR) and vascular endothelial growth factor (VEGF). These drugs have been approved as first, second and third line therapies for metastatic CRC (mCRC) and the advent of target-specific cancer therapeutics has remarkably improved the outcomes of patients with CRC. The molecular mechanisms underlying the clinical response to these drugs are not fully understood, although recent studies have elucidated the effect of intracellular signaling pathways involving in particular RAS/RAF/MAPK signaling on the safety and efficacy of target-specific drugs. Activating mutations of KRAS and BRAF genes are genetic events in tumorigenesis and these mutations are implicated as predictive factors in determining response, in particular to anti-EGFR drugs, and additional data suggest that other EGFR downstream pathways such as PI3K/PTEN/Akt or JAK/STAT are also important when considering mechanisms of EGFR antibody resistance. Recently the European Medical Agency (EMEA) approved the use of the mAb Panitumumab (December 2007) and approved a license extension for the use of the mAb Cetuximab in combination with chemotherapy as first-line treatment (October 2008) in mCRC patients with no mutations in the codon 12 and 13 of KRAS gene. The predictive value of KRAS mutations in the treatment of CRC has been very useful to clinicians and patients in terms of decision making, avoiding toxicities, and decreasing financial burden. This success also encourages researchers to find new markers with the same strong predictive value. Future studies also need to identify patterns of multiple mutations to further increase the power of patient selection for anti-EGFR therapy. These advances allow us to truly enter a new and exciting era of individualized therapy in oncology. Here we review the molecular basis of EGFR-targeted therapies and the resistance to treatments conferred by KRAS and other gene mutations as well as the laboratory methods used to detect all these genetic variations.