This report provides a graphical overview of historical trends in discretionary budget authority (BA) from FY1976 through FY2014, preliminary estimates for FY2015 spending, and the levels reflecting the President's proposals for FY2016 through FY2020 using data from the FY2016 budget submission released on February 2, 2015. Spending in this report is measured and illustrated in terms of discretionary budget authority as a percentage of gross domestic product (GDP). Measuring spending as a percentage of GDP in effect controls for inflation and population increases. A flat line on such graphs indicates spending that increased at the same rate as overall economic growth. Functional categories (e.g., national defense, agriculture, etc.) provide a means to compare federal funding for activities within broad policy areas that often cut across several federal agencies. Subfunction categories provide a finer division of funding levels within narrower policy areas. Budget function categories are used within the budget resolution and for other purposes, such as possible program cuts and tax expenditures. Spending caps and budget enforcement mechanisms established in the Budget Control Act of 2011 (P.L. 112-25; BCA) strongly affected recent budget cycles. Congress modified BCA caps for FY2013 as part of the fiscal cliff deal and modified caps for FY2014 and FY2015 through the Murray-Ryan agreement (Bipartisan Budget Act of 2013; BBA; H.J.Res. 59; P.L. 113-67). The BCA set discretionary spending caps on defense (budget function 050) and non-defense funding that are lowered to achieve a portion of spending cuts according to a formula in the BCA. The lowering of caps was turned off for FY2014 and FY2015 by the Murray-Ryan agreement. The estimated lowered defense cap for FY2016 is $523 billion, slightly above the FY2015 cap of $521.3 billion. The estimated non-defense lowered cap for FY2016 is $493.0 billion, close to the FY2015 cap of $492.4 billion. The Obama Administration, in its FY2016 budget submission, proposed raising caps to accommodate higher spending on domestic and military priorities. As the 114th Congress prepares to consider funding levels for FY2016 and beyond, past spending trends may help frame policy discussions. For example, rapid growth in national defense and other security spending in the past decade has played an important role in fiscal discussions. The American Recovery and Reinvestment Act of 2009 (P.L. 111-5; ARRA), a stimulus measure enacted after a sharp economic downturn, funded sharp increases in spending on education, energy, and other areas. Since FY2010, however, base defense discretionary spending has essentially been held flat and non-defense discretionary spending has been reduced significantly. The base defense budget excludes war funding (Overseas Contingency Operations/Global War on Terror). This report provides a starting point for discussions about spending trends. Other CRS products analyze spending trends in specific functional areas.