Congress rarely adds or subtracts a major new weapons system in votes on the floor. This results partly from lack of time but mainly from lack of expertise among members of Congress. Most congressmen serve on committees that deal with matters other than defense. Congressmen elected from districts with special economic interests are likely to gravitate to committees that have some influence over those interests. Congress can involve already existing groups in government decisions by making them part of a new procedure. Congressmen have recourse to procedure not only to mask their votes and achieve objectives without confronting the executive, but also to protect themselves politically. Congress is essentially a political institution and responds primarily to political stimuli. Congress as an institution is conservative, cautious and reluctant to initiate change. On arms control, as on many other subjects, the direct role that Congress can be expected to play is limited.