Different aspects of human-machine interaction are discussed in the five brief papers that comprise this report. The first paper, "Some Observations on Mental Models," discusses the role of a person's mental model in the interaction with systems. The second paper, "A Psychologist Views Human Processing: Human Errors and Other Phenomena Suggest Processing Mechanisms," discusses the differences between conventional digital processing structures (the Von Neumann machine) and the mechanism of the human. The third paper, "Steps toward a Cognitive Engineering," shows how analysis of error can lead to design principles. The fourth paper, "The Trouble with UNIX," is an informal critique of the UNIX operating systems. The final paper, "The Trouble with Networks," describes some of the computer interactions that resulted from the distribution of the fourth paper. (Author/LMM)