Cholangiocarcinoma (CCA) is a malignancy likely originating from the biliary epithelium, with an increasing incidence [
1‐
3]. Cholangiocarcinoma can occur at multiple points along the biliary tree, and is subtyped based on anatomic criteria into intrahepatic, perihilar, and distal [
2,
3]. Several risk factors have been associated with development of this tumor including hepatitis B and C infection, liver fluke infection, biliary stone disease, congenital biliary cysts, and underlying primary sclerosing cholangitis [
2‐
4]. However, the vast majority of patients have no identifiable risk factors. While evidence continues to accrue that the subtypes of CCA have unique molecular signatures (and likely represent very different cancers), the rarity of the tumor and the paucity of treatment trials has translated into a grouping of these tumors into “biliary tract cancers” (BTC) [
5,
6]. Gemcitabine and platinum-based combination chemotherapy have been defined as the standard first-line chemotherapy for unresectable or metastatic BTC [
7]. Unfortunately, the benefit of first-line therapy is limited, and for those patients that progress, no standard second-line chemotherapy has yet been established. Surgical resection remains the mainstay of treatment; however, even in patients with apparently resectable disease, recurrence rates are approximately 70% and 5-year survival a mere 30% [
8‐
10]. Adjuvant trials have examined several treatment paradigms including systemic chemotherapy and chemoradiotherapy. The results have been disappointing, with either no or very minimal improvements in overall outcomes [
11‐
13]. SWOG S0809 is a recently reported single-arm, phase 2 trial examining outcomes in patients with BTC treated with adjuvant gemcitabine and capecitabine, followed by capecitabine plus radiotherapy. R1 patients had similar survival as those with R0 disease, and overall survival (median 35 months) was favorable suggesting some benefit from this therapy; however, no control group was included which limits conclusions from this trial [
11]. In addition, two large adjuvant chemotherapy trials have been reported as abstracts and are awaiting final publication. These included the PRODIGE12-ACCORD18 trial and the BILCAP trial [
12,
13]. PRODIGE12-ACCORD18 was a multicenter, randomized, phase 3 trial that evaluated adjuvant gemcitabine and oxaliplatin versus observation alone following resection of BTC. The primary endpoint in this trial was recurrence free survival, with no difference in the study groups noted. Even when subgroup analysis was completed by tumor type, no positive findings were noted [
12]. The BILCAP trial was a multicenter, randomized, phase 3 trial that evaluated adjuvant capecitabine versus observation in patients with all the types of BTC. Comparing the groups, there was a notable increase in median overall survival from 36 months in the control group to 51 months in the capecitabine arm; however, this did not reach statistical significance in the intention to treat analysis. In per protocol analysis, the difference did reach significance [HR 0.75, (5% CI 0.58, 0.97;
p = 0.028)], leading the investigators to recommend consideration of adjuvant capecitabine for all resected BTC; however, this remains controversial given the results of the intention to treat analysis [
13]. Thus, there remains a critical need to develop effective therapies for the treatment of this lethal disease. Therapeutic advances will require additional insights regarding the molecular mechanisms of biliary carcinogenesis and tumor progression. Some progress has been made with the identification of isocitrate dehydrogenase (IDH) mutations and fibroblast growth factor receptor 2 (FGFR2) fusions as drivers in small subsets of intrahepatic cholangiocarcinoma tumors [
5,
14‐
16]. Further investigation has identified the Hippo pathway as a pathway of interest, as several investigators have demonstrated activity of Hippo pathway components in CCA [
17‐
24]. Herein, we review the Hippo pathway, regulation of its effector protein YAP, and the evidence for a role in CCA.