Abstract: An expandable reamer apparatus and methods for reaming a borehole are disclosed, including at least one laterally movable blade carried by a tubular body selectively positioned at an inward position and an expanded position. The at least one laterally movable blade, held inwardly by at least one blade-biasing element, may be forced outwardly by drilling fluid selectively allowed to communicate therewith or by at least one intermediate piston element. For example, an actuation sleeve may allow communication of drilling fluid with the at least one laterally movable blade in response to an actuation device being deployed within the drilling fluid. Alternatively, a chamber in communication with an intermediate piston element in structural communication with the at least one laterally movable blade may be pressurized by way of a movable sleeve, a downhole turbine, or a pump.