We have evaluated several square pore microchannel plates (MCP's) (25 mm MCP's with 85 micrometers diameter pores, 50:1 channel length to diameter (L/D) ratio, and 46 mm MCP's with 25 micrometers pores, 80:1 L/D ratio) from Philips. Measurements of the grain and pulse height distribution (PHD) vs voltage, PHD vs angle, background rate, flat field, quantum detection efficiency (QDE) vs angle, wavelength retarding field were made on these MCP's. The gain levels reach 2 - 3 X 107, with PHD's of < 55%, and background rates of < 0.5 events cm-2 sec-1. Flat field measurements show the 25 micrometers square pore MCP's to have periodic modulation, but the 85 micrometers square pore MCP's to have no measurable modulation. The difference is thought to be due to the MCP stacking configurations. The QDE as a function of wavelength for the square pore MCP's is not markedly different from that of normal uncoated round pore MCP's. The only significant difference is that the QDE variation with angle is much more rapid for the 25 micrometers square pore MCP's. Microscope examination reveals that the pore alignment is quite good for the imaging quality square pore MCP's. Low radioactivity MCP's from Galileo with an 80:1 L/D ratio, 10 micrometers pores, and a 32 mm active area were also tested as a stacked back-back pair. Background events were uniformly distributed over the field of view with an average rate of 0.063 events cm-2 sec-1. Surrounding the detector chamber by lead shield blocks reduced the background rate to only 0.028 events cm-2 sec-1 which is only a factor of 2 to 3 higher than the expected cosmic ray rate.