This monograph is the result of a course given to graduate students and to the faculty of the Dept. of Medical Physics and Biophysics of Nijmegen University, Nijmegen, The Netherlands, in the fall of 1984 and 1985. The course was intended to put together experi ment, theory, and analysis methods in order to study neural in teraction and coding in the brain. The following pages give a survey of neural interaction and its experimental substrate: cor related neural activity. The basic reason for restricting myself to vertebrate brains was to keep the material concise. As the text developed, however, it became more of a review, than a research monograph, in the attempt to balance theoretical and experimen tal aspects in brain research. Consequently, the book can be read from various points of view: that of requiring an overview of theories and theoretical principles, or an overview of experimental studies in neural interaction and the methods that can be used, or with the conviction that theory and experiment cannot be separat ed. In the latter case the book should be read from beginning to end. A way to read through the theoretical sections and the ex perimental sections of the book is presented in the following flow chart; Theory: /Chap. 2 -Chap. 4 -Chap. 5 ___ ~ Introduction -+ Chap. 1 \, Chap. 10 -+ Chap. 14 Experim~Chap. 3 -Chap. 6 -Chap. 7 -Chap. 8 ~ Chap.