The earth, viewed through the window of an airplane, shows a regularity and reptition of features, for example, hills, valleys, rivers, lakes, and forests. Nevertheless, there is great local variation; Vermont does not look like Utah. Similarly, if we rise above the details of a few programming languages, we can discern features that are common to many languages. This is the programming language landscape; the main features include variables, types, control structures, and input/output. Again, there is local variation; Pascal does not look like Basic. This work is a broad and comprehensive discussion of the principal features of the major programming languages. A Study of Concepts