A study of contact fatigue in silicon nitride is reported. The contacts are made using WC spheres, principally in cyclic but also in static loading, and mainly in air but also in nitrogen and water. Damage patterns are examined in three silicon nitride microstructures: (i) fine (F)-almost exclusively fully developed cone cracks; (ii) medium (M)-well-developed but smaller cone cracks, plus modest subsurface quasi-plastic damage; (iii) coarse (C)-intense quasi-plastic damage, with little or no cone cracking. The study focuses on the influence of these competing damage types on strength as a function of number of contacts. In the F and M microstructures strength degradation is attributable primarily to chemically assisted slow growth of cone cracks in the presence of moisture, although the M material shows signs of enhanced failure from quasi-plastic zones at large number of cycles. The C microstructure, although relatively tolerant of single-cycle damage, shows strongly accelerated strength losses from mechanical degradation within the quasi-plastic damage zones in cyclic loading conditions, especially in water. Implications concerning the design of silicon nitride microstructures for long-lifetime applications specifically in concentrated loading, are considered.