Yakima County centers on the Yakima Valley and extends from the summits of the Cascades to the within a few dozen miles of the Columbia, an incredible range of environmental conditions.

The county seat and largest city on the east slope is Yakima, whose prominence is due to a decision of the railways in the last century. The former chief point, also originally called Yakima, became Union Gap. Other major communities include Sunnyside and Toppenish, headquarters for the Yakama Indian Nation.

Between the western and eastern ends of the county, there is a climatic shift to very dry conditions typical of the Columbia Basin in Eastern Washington.