A new approach (ThinZag®) for scaling Nd:YAG solid-state slab lasers to high power levels is described. Using an innovative optical configuration, thin slabs of solid-state gain material are immersed in a flowing cooling fluid, resulting in improved thermal management. Measurements at the ~1 kW level were conducted to compare Nd:YAG ceramic with Nd:YAG crystalline slabs. The ceramic slabs demonstrated comparable laser output power relative to the more standard crystalline slabs and demonstrated better optical properties, which result in improved medium uniformity. Performance results from our nominally 5 kW and 15 kW modules are also presented. The 15 kW modules represent basic building blocks for the Joint High Power Solid State 100 kW Laser (J-HPSSL) currently being designed and developed by Textron Defense Systems. Recent measurements of three nominally 15 kW modules operating as a single aperture power oscillator where outputs to 30 kW with beam quality of 3.3 XDL were measured are also included. The reported powers were for operation below maximum pump rate of the device.