Background: PD-1 signaling in the tumor microenvironment dampens immune responses to cancer and blocking this axis induces anti-tumor effects in several malignancies. Expression of PD-L1 has been associated with poorer outcomes in certain cancers and may predict response to anti-PD-1 agents. Clinical studies of PD-1 blockade are only now being initiated in pediatric patients and little is known regarding PD-L1 expression in common childhood cancers. We characterized PD-L1 expression and Tumor Associated Immune Cells (TAIC, lymphocytes and macrophages) in common pediatric cancers. Methods: Whole slide sections (N = 91) and tissue microarrays (N = 529 cores) were evaluated by IHC for PD-L1 expression and for the presence of TAIC. A subset of 60 tumors was further assessed by multiplex immunohistochemistry (IHC) for CD3, CD4, CD8, CD45RO, PD-1, and FoxP3 expression. Results: Nine percent (39/451) of evaluable tumors expressed PD-L1 in at least 1% of tumor cells. Highest frequency histotypes were Burkitts lymphoma (80%, 8/10), glioblastoma multiforme (36%, 5/14), and neuroblastoma (14%, 17/118). PD-L1 staining was associated with inferior survival among neuroblastoma patients (p = 0.0025), including those with high-risk disease (p = 0.0027). Seventy-four percent of tumors contained lymphocytes and/or macrophages. Macrophages were frequently found to be PD-L1 positive (65%) and were significantly more likely to be found in PD-L1 positive than negative tumors (p < 0.001). By multiplex IHC, infiltration of CD8+ T cells was seen in 77% of 60 evaluable samples. Fox-P3 and PD-1 were expressed in 42% and 47% of the samples, respectively. Conclusions: A subset of diagnostic pediatric cancers show PD-L1 expression, while a much larger fraction demonstrate infiltration with tumor associated lymphocytes and macrophages. PD-L1 expression may be a biomarker for poor outcome in high-risk neuroblastoma. Macrophages are present in a subset of tumors and may play an immune modulatory role. Further preclinical and clinical investigation will define the predictive nature of PD-L1 expression in childhood cancers both at diagnosis and after exposure to chemoradiotherapy.