Index-antiguided (IAG) waveguide is promising for high-power application due to simple step-index structure and the property to introduce higher waveguide loss for higher-order-modes (HOMs). The loss ratio between fundamental mode (FM) and 1st HOM is fixed at 4 for IAG planar waveguides and 2.54 for IAG fibers. Since 2006, robust single-transverse-mode oscillation has been reported both in IAG fibers with diameter up to 400 µm and IAG planar waveguides with 220 µm core width. Although IAG waveguides seem to be promising for large-single-mode operation, the main challenges are the low output efficiency in end-pumped IAG lasers and the effect of HOM oscillation. The objective of the research is to solve the above two challenges. In this dissertation, both core and cladding pumping schemes are numerically studied, demonstrating that cladding pumping is not efficient in IAG fiber lasers due to both index-antiguiding effect and large core size, and core pumping should be implemented instead. To achieve high output efficiency, laser parameters need to be carefully designed to satisfy that the gain length Lgain =1/σapN is much smaller than the decay length Ldecay = -lnRoc/2αs, and the fiber length should be chosen such that Lgain ≤ L << Ldecay. This conclusion works well to explain the low output efficiency issue for the published experimental results. In addition, the investigation of transverse mode competition effect in IAG lasers shows that in order to maintain single-mode operation, the maximal extraction efficiency of single-fundamental-mode is suppressed to 66.7% and 50% (from 100%) for IAG slab and fiber lasers, respectively. This low efficiency is due to the small loss ratio (2.54 for IAG fibers and 4 for IAG planar waveguides) in IAG waveguide lasers.