Waveguide arrays with slots in the broad face use either longitudinal shunt slots displaced from the centerline or inclined series slots on the centerline. Despite the established popularity of the shunt-displaced slot, a case can be made for the superiority of the series-inclined slot. The resonant length of the series-inclined slot is substantially independent of the inclination, and the cross-polarized side lobes are easily reduced by a simple filter. For the shunt slot, resonant-length variations and unwanted side-lobes caused by the offsets are problems less easily solved. Some general features of four types of slotted waveguide arrays are discussed: (1) untransposed series or shunt slots, (2) transposed series or shunt slots, (3) interlaced series and shunt slots firing forward, and (4) interlaced series and shunt slots firing backward. Typical grating lobe spectra are shown and the relationship of the slotted linear array to the optical diffraction grating is discussed. The four types of slotted waveguide arrays presented are special cases of Rayleigh's beam-forming phased array in which the diffraction grating is "blazed" so that all the light falls in a particular direction. Slotted array design techniques are reconsidered briefly and "leakance" characteristics are shown for the one-third constant plus two-thirds cosine squared distribution (for different powers in the termination). The Fourier transform character of the input SWR versus frequency pattern is noted. Auxiliary fittings for slotted arrays (parallel plates, chokes, prisms, and horns) are also reviewed. Series-slotted waveguide arrays have been built by AIL for K_{u} -, X-, and S-bands, and for 332 Mc, where the array is 54 feet long with twenty-six 18-inch slots in a 26 by 13 inch waveguide. Two such arrays (side by side) were developed for an aeronautical glide-slope application. There is special interest in all of these arrays because of their use at or near the broadside beam-firing angle. Measurements of patterns made at or near the zero squint angle showed satisfactory beam formation with side lobes below -20 db and cross-polarized lobes below -27 db. Input SWR's were less than 2 to 1. Isolation between adjacent antennas was better than -45 db.