Limited data exists on atypical lipomatous tumor (ALT)/well-differentiated liposarcoma (WDL) and dedifferentiated liposarcoma (DDLPS) in children and young adults.Cases of ALT/WDL/DDLPS arising in patients <40 years were collected from multiple institutional and consultation archives.116 cases of ALT/WDL (75) and DDLPS (41) were identified, representing less than 5% of these tumors seen at our institutions over this time period. The patients (59M/57F) ranged from 8 to 40 years. Sites included deep central (abdomen/retroperitoneum/pelvis/groin) (n=60), extremity (n=42), trunk (n=5) head/neck (n=8), and mediastinum (n=1). Dedifferentiated patterns included: high-grade pleomorphic sarcoma, myxofibrosarcoma-like, heterologous rhabdomyoblastic differentiation, low-grade spindle cell sarcoma, and homologous lipoblastic differentiation. 41 patients experienced a local recurrence, and 11 patients with DDLPS developed metastasis. ALT arising in the extremities had lower recurrence rates than deep central WDL (5-year recurrence-free survival 88.9% vs 59.0%; p=0.002), while patients with deep central DDLPS experienced significantly more adverse events compared to WDL at this site (5-year event-free survival 11.9% vs. 59.0%) (p<0.0001). 7 (of 8) head/neck tumors had follow-up available; 5 recurred, and one patient (DDLPS) with recurrence also experienced a metastasis. The single mediastinal tumor (DDLPS) recurred and metastasized.ALT/WDL and DDLPS occurring in patients < 40 years is rare but exhibits similar morphologic features to its counterparts in older adults, including potential for heterologous and homologous dedifferentiation in the latter. Although case numbers are limited, tumors arising in the head and neck exhibit high rates of adverse events, suggesting that classification as WDL rather than ALT is more appropriate.