The efficiencies as well as other spectral properties of near-UV, violet or blue light emitting diodes (LEDs) are too critical to be neglected when designing phosphors for white light emission in solid state lighting. The efficiency, in particular, limits which phosphors are appropriate and which mechanisms should be explored further. In this talk, we will present efficiency data from state-of-the-art LEDs (Figure 1) and discuss their implications for phosphor design. Given the spectral variations of LED efficiencies shown in Figure 1, we will consider limitations imposed on luminescence mechanisms such as absorption and emission processes, Stokes shift, and radiative lifetime, and high flux operations. Specifically, we will compare f→f, f→d, and d→d absorption mechanisms and use arguments based on oscillator strength to demonstrate why phosphors that rely on f→f absorption may not be good options. Stokes losses are unavoidable for any phosphor, but need to be minimized for overall energy efficiency. At the same time, designing a blue phosphor for either near-UV or violet LEDs is rather too difficult because of the required small Stokes shift for a broadband phosphor. Even if an efficient blue phosphor could be found, its emission is re-absorbed by the green and red components leading to lower efficacies. Another important consideration is the flux level encountered by the phosphor layer in LEDs. For a 1 mm2 LED operating at 35 A/cm2, the irradiance for a chip-level-converter is on the order of 0.5 W/mm2 (assuming a 1 W chip with 50% energy efficiency). This is significantly higher than traditional fluorescent technology where the irradiance is ∼10−4 W/mm2 for a typical T8, 4 ft. lamp operating at 50 kHz.[1]At such high flux densities, a phosphor needs to have either a short radiative lifetime or it must be removed from the chip and used in a remote configuration in order to avoid ground state depletion. With laser-activated phosphors becoming more popular, these lifetime considerations will become even more crucial. We will also discuss the need for narrowband red phosphors and briefly evaluate the known options such as f→f transitions from trivalent lanthanides, d→d transitions from Mn4+ or Cr3+, and relatively narrow band emission due to d→f transitions from Eu2+in some nitride hosts, and, of course, quantum dots. Finally, simulations comparing the spectral efficiencies and color rendering quality of both near-UV and blue pumped white light sources will be presented. Optimal choices for spectral properties of phosphors as well as LEDs will be discussed. [1] A. Piquette, W. Bergbauer, B. Galler, and K. C. Mishra, ECS J. Solid State Sci. Technol., 5, R3146 (2016). Figure 1