Soft tissue sarcomas represent an important chapter of pediatric oncology, accounting for about 10% of all malignancies in childhood. The aim of this work is to summarize the clinical, histological, immunohistochemical and genetic features of the most frequent soft tissue tumors presenting in infants and in young subjects before the age of 10. For each entity, the most relevant data regarding prognosis and treatment will be summarized, and the most important morphological and immunohistochemical features will be reported. The most frequent myofibroblastic tumors, fatty tumors and skeletal muscle tumors occurring in infancy and adolescence will be described in this first part. The aim of this work, mainly based on a practical approach, is to help perinatal and pediatric pathologists in the diagnosis of a group of tumors that are diagnostically challenging, due to their rarity, the contemporary expression of multiple immunohistochemical markers and frequent lack of known genetic abnormalities.