An electron source technology compatible with micro-fabricated structures would enable many new devices ranging from x-ray sources to a new generation of high frequency vacuum electron devices. One well-known candidate source is the field emitter array. Field emission allows electrons to be removed from solids without first raising their energy (eg by heat or radiation), making them very convenient to use. The non-linear I-V characteristic allows row-column addressing. Local emission current densities over 10@super 8@ A/cm@super 2@ are possible, limited by space charge. Field emitter arrays, multiple field emission structures together with local electrodes used to produce the extraction field, can be fabricated using a number of standard and exotic patterning methods. Individual cells in an array typically have dimensions on the order of 1 micron. Transit through the high field region takes less than 1 pS, potentially allowing THz emission modulation. Average current densities over 10@super 3@ A/cm@super 2@ and total currents over 100 mA have been demonstrated. However, producing these outstanding results is not easy. One might assume that N field emitters in parallel might produce N times the current that can be produced by a single tip. But variations among emitters typically allow only a small fraction of the emission sites to produce current, and the fraction tends to decline with the number of emission sites. The total current is typically no more than 10@super 3@ times the maximum current from a single emitter, even when 10@super 5@ or 10@super 6@ emitters are present. The poor uniformity is not surprising, given that the field emission current is sensitive to both the shape of the emitter and the density and energy of electrons at the surface. I will discuss some strategies for improving the total emission current.