The intensity distribution of laser beams in the focal plane of a focusing optic is important because it determines the laser-matter interaction process. The intensity distribution in the focal plane is determined by the incoming laser beam intensity and its wavefront profile. In addition to the intensity distribution in the focal plane, the intensity distribution near the focal plane is also important. For a simple laser beam having a Gaussian or flat-top intensity profile, the intensity distribution near the focal plane can be analytically described. In many cases, however, the laser beam profile cannot be simply described as either Gaussian or flattop. To date, many researchers have attempted to characterize laser beam propagation using a simple metric for laser beams having an arbitrary beam profile. With this trial, researchers have devised a beam quality (or propagation) factor capable of describing the propagation property of a laser beam, especially near the focal plane. Although the beam quality factor is not a magic number for characterizing the beam propagation, it can be widely applied to characterizing the propagation of a laser beam and is also able to quickly estimate how small the size of the focal spot can reach. In this chapter, we start by describing the spatial profile of laser beams. In Section 2, the derivation of the spatial profile of laser beams will be reviewed for Hermite-Gaussian, Laguerre-Gaussian, super-Gaussian, and Bessel-Gaussian beam profiles. Then, in Section 3, the intensity distribution near the focal plane will be discussed with and without a wavefront aberration, which is another important parameter for characterizing laser beams. Although the Shack-Hartmann wavefront sensor is widely used for measuring the wavefront aberration of a laser beam, several other techniques to measure a wavefront aberration will be introduced. Knowing the intensity distributions near the focal plane enables us to calculate the beam quality (propagation) factor. In Section 4, we will review how to determine the beam quality factor. In this case, the definition of the beam quality factor is strongly related to the definition of the radius of the intensity distribution. For a Gaussian beam profile, defining the radius is trivial; however, for an arbitrary beam profile, defining the beam radius is not intuitively simple. Here, several methods for defining the beam radius are introduced and discussed. The experimental procedure for measuring the beam radius will be introduced and finally determining the beam quality factor will be discussed in terms of experimental and theoretical methods.