This thesis explores the potential role of PIK3CA mutations as predictive and prognostic biomarkers in both colorectal cancer (CRC) and non-small cell lung cancer (NSCLC). Cancer researchers started to focus on this gene, because somatic alterations in PIK3CA were linked to tumorigenesis. This review covers 22 different studies that looked at the influence of PIK3CA on the outcome of both targeted therapies and adjuvant treatment with aspirin, as well as its influence on the natural course of CRC and NSCLC. The majority of research groups found a significant negative association between PIK3CA mutations and the response to cancer therapy with anti-EGFR agents. In contrast, PIK3CA mutations had a positive effect on CRC patients using aspirin as an adjuvant treatment. The true value of PIK3CA as a prognostic biomarker is, however, still unclear, due to inconsistencies between research results. As such, further research is required before PIK3CA can be conclusively defined or refused as a prognostic biomarker.