The adoption and use of health information technology (IT) by health systems in ambulatory care can be an important driver of care quality. We examine recent trends in health IT adoption by health system-affiliated ambulatory clinics in the context of the federal government's Meaningful Use and Promoting Interoperability programs. We analyzed a national sample of 17,861 ambulatory clinics affiliated with 1711 health systems, using longitudinal data (2014-2016) from the HIMSS Analytics annual surveys. We used descriptive analyses and linear probability models to examine the adoption of electronic health records (EHRs), as well as 16 specific functionalities, at the clinic level and the system level. We compared the differential trends of adoption by various characteristics of health systems. We find that the adoption of an EHR certified by the Office of the National Coordinator for Health IT (ONC) increased from 73% to 91%. However, in 2016, only 38% of clinics reported having all 16 health IT functionalities included in this study. Small health systems lag behind large systems in ambulatory health IT adoption. Patient-facing functionalities were less likely to be adopted than those oriented toward physicians. Health information exchange capabilities are still low among ambulatory clinics, pointing to the importance of the ONC's recent Promoting Interoperability initiative. The relatively low uptake of health IT functionalities important to care improvement suggests substantial opportunities for further improving adoption of ambulatory health IT even among the current EHR users.