Soil horizons reflect soil processes and convey information about past and present soil conditions. The identification and delineation of soil horizons are affected by lateral and vertical variation in soil properties. Early studies focused on the variation of horizon thickness and the waviness of horizon boundaries, but did not consider within-horizon lateral and vertical variation. Here we review studies that investigated variation in the master horizons O, A, E, B, and C. We summarize what is known about soil horizon variation, quantify the variation in different horizons, and investigate whether the variation increases or decreases with depth. The variation within horizons differs among soils, and the magnitude of the variation varies for different soil properties. Variation within soil horizons or laterally within a few square meters may be considerable, and the within-horizon variation changes with depth. Horizon thickness does not seem to be related to the variation of soil chemical and physical properties within the horizon, i.e., thicker horizons do not necessarily have higher variation in their soil properties. Three case studies are presented: Spodosols and Histosols (Russia), Alfisol and Mollisol (USA), and Oxisol (Brazil). Factors that affect the within-horizon variations include landscape position, parent material, vegetation, fertilization, tillage, drainage, and time. The vertical distribution of soil properties can be quantified using soil depth functions. Digital soil morphometrics techniques can assist in the quantification of two-dimensional soil profile properties and variations.