The incidence of cancer, diabetes mellitus, cardiovascular and some other diseases continues to increase and there are still no sufficiently successive healing and preventive strategies. Insulin-like growth factors (IGFs) play important roles during entire life and, depending on the biochemical pathway in which they are involved, they may be regarded as growth factors, hormones or cytokines. IGFs exert their functions after binding to specific receptors and their availability is regulated by IGF-binding proteins. Altered activity in the IGF signaling pathways is connected with diseases, but data are controversial as both beneficial and harmful effects of IGFs were recorded. Traditional approaches to study biochemical pathways, mostly based on the synthesis and clearance rates, are not sufficient. Post-translational protein modifications and associations with other biomolecules may represent an intermediate level of control, which deserves more attention. This book provides an overview of the post-translational events which affect the IGF system, offering experimental data to document structural and functional changes which, as a final consequence, have altered IGF activity.