We review our recent work toward designing large mode area fibers for high power applications. We show that appropriately designed doped multi-mode fibers can be used to provide robust single-mode output when used in fiber laser cavities. Single-mode (SM) fiber mode field diameters of ~35µm are demonstrated with record SM pulse energies of 0.5mJ at 1550nm with a repetition rate of 200Hz. Energies approaching 1mJ are obtained with a slight compromise in mode quality. A modification in the laser cavity results in a passively modelocked laser giving femtosecond pulses with nanojoule energies.