This paper covers briefly the theory of tire-road friction, coefficient of friction measurement techniques, and the vagaries of tire-road friction as they relate to critical speed estimation. A literature review of tire-road friction studies was conducted to identify the primary factors effecting the tire-road coefficient of friction. Background information is presented covering general definitions and the connection between the basic critical speed formulas and the coefficient of friction. The primary components of tire-road friction, adhesion and hysteresis, are discussed along with minor effects such as tearing, wear, waves, and roll formation. Common coefficient of friction field measuring techniques are described, including the skid-to-stop test and drag sled. Influential factors such as tire characteristics, tire inflation pressure, road conditions, and dynamic factors are reviewed. Important dynamic factors are listed and the connection between longitudinal and lateral friction is discussed. Overwhelmingly, the literature indicates that the coefficient of friction is the function of many variables and that it is the most ubiquitous factor affecting speed estimates when the critical speed formula is used. Unfortunately, there appears to be no consensus regarding the appropriate value or measurement technique for the tire-road coefficient of friction used to estimate critical speed.