This paper presents a general framework for designing optimal transmit spectra for communication services dominated by crosstalk --- for example Digital Subscriber Lines (DSLs) and wireless LANs. Using the channel, noise, and interference transfer functions, we set up and solve an optimization problem to maximize the joint channel capacity. We employ joint signaling techniques and optimal power distribution to yield significant gains in bit rates (or performance margins). Furthermore, by design, the spectra are spectrally compatible with existing neighboring services. The framework is quite general; it does not depend on the exact choice of the modulation scheme, for example. It is also extremely simple and of low computational complexity.