Geophysical evidence suggests that some faults are frictionally strong, in agreement with laboratory measurements of quasi-static frictional strength (μ ≈ 0.6-0.8) for many crustal materials; whereas others studies have found that some faults are weak when compared to laboratory friction values (μ < 0.5). It has also been well documented that fault materials undergo a significant dynamic reduction in frictional strength when the sliding velocity accelerates to earthquake slip rates (on the order of meters per second). In this talk I will review our current understanding of fault strength evolution during the seismic cycle, then I will present results from two recent laboratory studies where we attempt to elucidate some of the dominant controls on fault strength both before and after an earthquake has occurred. Firstly, I will present results from a study where we investigate how geological heterogeneity in fault zones affects fault strength and stability; we find that heterogeneous faults are considerably weaker and more frictionally unstable than compositionally identical faults with an initially homogeneous structure. Then I will present results from some high-velocity friction experiments where we investigate how faults recover their strength after experiencing dynamic weakening during a seismic slip event. Our findings show that fault strength recovery (healing) occurs rapidly after high-velocity slip, which has important implications for our understanding of rupture dynamics and earthquake recurrence.