This paper explores the constraints on the design of twowire repeaterless digital subscriber loop (DSL) systems. Broadly categorized, the design depends on the technical feasibility of the approach used to achieve two-wire transmission, constraints related to compatibility with other systems sharing the same cable, and immunity to central office noise. Each of these varies With the choice of system parameters including the transmission rate, transmit power, choice of line codes, etc. Technical feasibility is evaluated by computer simulation studies. Compatibility with other systems is explored by crosstalk calculations. Noise immunity considerations, as they translate into digital line power levels, are also explored.