A hand-holdable steering mechanism is used as part of a medical device such as a catheter or an endoscope to allow movement of a steerable distal portion of the catheter or endoscope. The mechanism can include a housing and an actuation system. The actuation system is adapted for one-handed operation by a user, and it allows the user to achieve 360 degree articulation of the steerable distal portion. The actuation system can include a first actuator and a second actuator where the first and second actuators are movable with respect to each other. The first actuator can be at least partially disposed within the second actuator.