Abstract: Fiber lasers are capable of high average power at high efficiency, becoming the laser of choice for manufacturing and military applications. However, their small output aperture limits them to relatively low energy per fiber, due to nonlinearity at high intensity, and their small gain volume. The peak intensity limitation can be overcome by coherently adding pulses in time within a fiber amplifier, while adding the outputs of many fibers overcomes the energy limit. Amplifier bandwidth limits can be avoided by coherently adding multiple spectral slices to produce shorter pulses. I will present work being done at LBNL to develop coherently combined fiber laser systems, aimed at the Joule/kHz level for driving laser-plasma acceleration. Roughly half of this work is optical technology. The other half is sophisticated digital signal processing and controls, which provides robust stabilization of many-path optical interferometers at levels far below a wavelength.