This report provides summary data on the number of Senators and Members of the House who first entered Congress between the 64 th Congress (1915-1917) and the 114 th Congress (2015-2016). Since the convening of the 64 th Congress, 4,201 individuals have entered the House of Representatives for their first, or "freshman," terms as a Representative. An additional 28 have begun service as a Delegate or Resident Commissioner. During th e same period, 844 individuals began their first terms in the Senate. First-term membership is divided into two broad categories in each chamber: Members chosen prior to the convening of a Congress, and those chosen after a Congress convenes. The "pre-convening" category includes Members who were elected in the general election, and in any special elections held prior to the convening of a Congress. The 64 th Congress was chosen as the starting point for data collection because it was the first Congress for which Senators were chosen by direct popular election. This provides a single date upon which most Members in both chambers are chosen to serve prior to the convening of a Congress. In the Senate, the pre-convening category also includes any Senators who were appointed to the Senate prior to the convening of a new Congress. The "post-convening" category includes Members who joined either chamber after the convening of a Congress. Means by which seats may be filled by a post-convening Member in either chamber include special elections held after a Congress convenes or electoral challenges that result in a new Member being seated. In the Senate, a first-term Member may also join the chamber through appointment or special election. Members whose congressional service in one chamber is not consecutive are counted as first-term Members in the first instance of their service as a Member, if that term occurred between the 64 th and 114 th Congresses. For example, a Representative who served in the 87 th Congress (1961-1962), and 89 th Congress (1965-1966), but not the 88 th Congress (1963-1964), would be counted as a first-term Representative only for the 87 th Congress. Members with service in the House and Senate are listed in each capacity in which they served a first term, if those terms occurred between the 64 th and 114 th Congresses. The resulting data, combining pre-convening and post-convening first-term Members, provide a count of all Members who served a first term in the House or Senate. Data on pre-convening first-term Members provide partial insight into the extent of membership turnover in the House and Senate since 1915, and are discussed in greater detail below. Post-convening first-term Member data do not reveal clear patterns within individual Congresses, or over time. This is due in part to the wide range of reasons that a seat in the House and Senate may become vacant in the course of a Congress, and the circumstances under which it may be filled. Data describing the number and partisan breakdown of first-term membership in the Senate are provided in the " Data Tables " section in Table 6 . Data describing first-term Representatives are available in Table 7 . Table 8 in the same section provides information for Members of the House serving as a Delegate or Resident Commissioner. Data on pre-convening first-term Members provide insight into the extent of membership turnover in the House and Senate. Table 5 in the " Data Tables " section summarizes the number of pre-convening Members entering the House and Senate at the beginning of a new Congress, and as a percentage of the seats in each chamber. These data identify most of the turnover in each chamber, but they may not identify all of the changes in every Congress, since they only reflect the number of Members who served their first term in the chamber. Some Members who had prior service that is not consecutive may have been reelected to the House, or reelected or appointed to the Senate. In those circumstances, the data in Table 5 may understate the extent of change in some Congresses. Since the 64 th Congress, the average turnover in the House with each election has been 72 seats, or 16.57%. The election with the greatest change occurred in 1932, resulting in a turnover of 158 seats, or 36.32% of the Representatives between the 72 nd Congress (1931-1933) and the 73 rd Congress (1933-1934). The smallest pre-convening turnover among Representatives in the House occurred in the 101 st Congress (1989-1990), with a change in 30 seats, or 6.90%. Figure 1 provides a graphic representation of the percentage change in House membership between the 64 th Congress and the 114 th Congress. The data suggest that while there is no consistent pattern of change from Congress to Congress, the overall number of new, pre-convening, first-term Representatives has declined. This appears to be consistent with some academic findings that argue that the durations of Members' careers have been increasing in the past century. Table 1 provides data for the House on the number of seats and percentage change of the five Congresses that saw the greatest change in pre-convening Representatives between the 64 th and 114 th Congresses. With one exception, the 103 rd Congress (1993-1994), these changes occurred in Congresses convening prior to the 74 th Congress (1935-1936). Table 2 provides data on the number of seats and percentage change of the six Congresses that saw the least change between the 64 th and 114 th Congresses. The smallest pre-convening turnover among Representatives in the House occurred in the 101 st Congress (1989-1990), with a change in 30 seats, or 6.90%. All of the smallest changes occurred after the 89 th Congress (1965-1966). The distribution of greater changes occurring earlier in the period between the 64 th -114 th Congresses, and smaller changes happening in the later period may also support contentions regarding the duration of Representatives' careers. Data describing the number and partisan distribution of first-term Representatives are provided in Table 7 . Table 8 provides similar information for Members of the House serving as a Delegate or Resident Commissioner. As shown in Table 5 , in the " Data Tables " section, since the 64 th Congress, the average number of pre-convening first-term Senators each Congress has been 10. Table 6 shows that the 79 th Congress (1945-1946) produced the greatest change in membership with 33 new Senators, 34.38%, taking seats in the chamber in the course of the Congress. Figure 2 provides a graphic representation of the percentage change in first-term Senate membership between the 64 th Congress and the 114 th Congress. The data suggest that while there is no consistent pattern of change from Congress to Congress, the overall number of pre-convening, first-term Senators has declined since the 64 th Congress. Changes in Member career patterns in the Senate may explain some of the change. Table 3 provides data on the number of Senate seats and percentage change of the five Congresses that saw greatest change between the 64 th and 114 th Congresses. All of those Congresses occurred before the 87 th Congress (1961-1962). The smallest turnover of pre-convening Senators occurred in the 102 nd Congress (1991-1992), with a change of three seats. In the 73 rd Congress (1932-1933), a 15-seat change amounted to a percentage change of 15.63%, since the Senate had 96 seats. Table 4 provides data on the number of seats and percentage change of the seven Congresses that saw the least change between the 64 th and 114 th Congresses. Smaller changes appear to be more evenly distributed through the latter half of the Congresses observed. This may be explained in part by electoral patterns. While the entire House stands for election every two years, only one-third of the seats in the Senate are subject to election in the same period; barring change in membership for other reasons, this assures that two-thirds of Senate membership will remain unchanged. Data describing the first-term membership of the Senate are provided in Table 6 .