SDI: Strategic Defense Initiative is based around classic-style shoot 'em up action. It puts you in control of a satellite which must ensure that the wave of bombs aimed for the United States do not hit. Direction of motion and direction of fire are controlled independently - on the 16-bit versions you rotate the mouse left and right to direct the targeting, move it up and down to increase the range, and use the joystick to move the satellite and thus avoid any contact.
Each level is split into defensive and offensive stages. On defensive levels you must stop each unit as it heads towards the earth, a lá Missile Command. Offensive stages follow the old 'shoot it before it becomes a danger' philosophy, being free-flying shoot 'em up sections among a network of destructible (but worth saving) platforms.