This book presents the findings on interactions of microRNAs and mRNAs involved in gastrointestinal malignancies. Presented study includes in silico algorithms for microarray data preprocessing, microRNA targets prediction and data integration. The assessment of changes occurring in regulatory networks is additionally extended by significance of transcription factors affecting expression of both, microRNAs and mRNAs during carcinogenesis of esophagus, stomach, pancreas and colon.