Roots experience mechanical impedance due to the force required to displace soil particles as they elongate. Mechanical impedance (or soil strength) is the most ubiquitous constraint to root growth. Strong soil can be a serious agricultural problem, as the ability of the root system to access water and nutrients from the deeper soil layers is restricted (Barraclough and Weir 1988). Soil is a complicated material and its properties are affected by land management and also by the roots themselves. This chapter has two objectives: first, to explain how soil type, management, and water status affect soil strength and, second, to describe how roots are able to penetrate strong soil and how root traits enable good penetration of strong soils. The idea of soil strength is communicated in a number of ways and often “compact” or “hard” is used to describe soil that is difficult for roots to deform and penetrate. In this chapter we refer to strong and weak soil. To clarify our definition of strong and weak soil, we first describe its measurement in a way that is appropriate to root growth studies: “When you can measure what you are speaking about and express it in numbers, you know something about it,” Lord Kelvin, speaking to the Institution of Civil Engineers, May 3, 1883.