Background: Uterine carcinosarcomas (UCS) are rare, aggressive tumors accounting for 5% of all uterine cancers. Recurrence rates are high and 5-year survival is only 18-39%. HER2 is an emerging prognostic and therapeutic target in uterine cancer including UCS. Testing algorithms and platforms in breast and gastric cancers are well studied and validated, but optimal HER2 testing in uterine cancer is not yet established. We aimed to determine HER2 prevalence in UCS and the concordance of chromogenic in situ hybridization (CISH), immunohistochemistry (IHC), and next generation sequencing (NGS) platforms to aid in the development of UCS specific testing guidelines. We also evaluated the rate of downstream mutations and immune biomarkers that may affect response to HER2 directed therapy. Methods: Eight hundred and seventy-five UCS tumor samples (primary 81.4%; metastatic 17.1%; unknown 1.5%) were analyzed using NGS (592, NextSeq; WES, NovaSeq, Caris Life Sciences, Phoenix, AZ). A subset of tumors with HER2 positivity were tested with IHC (4B5, Ventana) and/or CISH (INFORM DUAL HER2 ISH Assay, Ventana) based on 2018 ASCO/CAP HER2 Breast Cancer guidelines. Amplification of ERBB2/HER2 by NGS used a copy number cut-off >6. PD-L1 expression was analyzed by IHC (SP142, positive cut-off ≥1%, Ventana). Tumor mutational burden (TMB) was measured by counting all somatic mutations found per tumor (TMB high cut-off > 10 mutations per Mb). Microsatellite instability (MSI) was tested by fragment analysis (FA), IHC and NGS. Results: Rates of HER2 positivity were 3.4% (28/820) by NGS. Among the IHC/CISH-tested cohort, 7.2% was positive (10/139) by IHC, and 19.5% (27/133) by CISH. In the 105 samples tested with both IHC and CISH, the concordance was 100%. Specifically, 9/105 patients (8.6%) were IHC+/CISH+ and 96 patients (91.4%) were IHC-/CISH-. The concordance between CISH and NGS (N = 127) was 90.6% (sensitivity 100% and specificity 89.3%). Common gene alterations in CISH HER2+ UCS tumors that may implicate resistance to HER2 targeted therapy included mutations in TP53 (100%), FBXW7 (22.2%), PIK3CA (33.3%), PIK3R1 (18.5%), PTEN (3.7%) and KRAS (3.7%) and gene amplification of KRAS (11.1%). CISH+ HER2+ UCS tumors had low immunotherapy biomarker prevalence (0% MSI-H, 0% TMB high, 7.7% PD-L1+). Conclusions: Increased HER2 positivity was detected via CISH testing compared to IHC and NGS, which may reflect the heterogeneity of HER2 amplification due to mixed histology between the sarcoma and carcinoma portion of the tumor. High concordance rates were observed between CISH and IHC. These testing platforms need to be validated by response to HER2 targeted therapies in order to develop UCS specific testing guidelines.