Samples of Topopah Spring tuff from Yucca Mountain, Nevada have been tested and analyzed. Laboratory tests conducted include standard uniaxial and triaxial compression tests, and special 'damage' tests in which samples are loaded to some proportion of their strength and analyzed with SEM microscopy. Based on the SEM analysis of the damaged samples, the micromechanics of rock deformation and failure in Topopah Spring tuff is determined. The results indicated that pores are the major initial microstructures in Topopah Spring tuff. Also, several mechanisms have been found for microcracking under compressive stresses, including pore crackling, the linking of pore cracks, and the formation of en echelon arrays of axial cracks. The macroscopic cracks tend to propagate in the locations with the highest pore density. The final failure of Topopah Spring tuff is due to shear localization near the peak stress. The microbuckling of crack-induced columns has been found to be the major mechanism for inducing shear localization. The heating of Topopah Spring specimens up to 200°C results in no significant microcracking.