1 Copyright © 2010, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 1 Interaction Data: Defnitions, Concepts and Sources John Stillwell University of Leeds, UK Adam Dennett University of Leeds, UK Oliver Duke-Williams University of Leeds, UK ABSTRACT This initial chapter has two aims. Firstly, it seeks to clarify defnitional and conceptual issues relating to the key interaction phenomena, migration and commuting, on which the authors concentrate in this book and for which they strive to obtain information to enhance their understanding of the processes that are taking place in the real world. The chapter explains the conceptual distinction between mi- grants and migrations, the importance of which becomes clear when the difference between transition and movement data is outlined, and it considers the alternative units of migrant measurement that are used such as individuals, wholly moving households and moving groups. Whilst migration tends to be measured over a period of time, typically a year, commuting is an activity that occurs on a much more frequent basis and consequently is usually measured as the numbers making a journey on one day. The chapter indicates how commuting to work and commuting to study are defned and measured. Secondly, the chapter contains the summary of an audit of interaction data sources, outlining the characteristics of the different types of data that are available from censuses, registers and surveys. Particular emphasis is placed on the former, the Census of Population, for which there are a number of data products providing migration and commuting counts at different spatial scales and disaggregated by various attributes; micro data are distinguished from macro data. However, the chapter also introduces a range of other interaction data sources such as the registers of National Health Service patients, the Pupil Level Annual School Census, the databases of the Higher Education Statistics Agency, various national level surveys such as the Labour Force Survey and the International Passenger Survey. In some cases, the data are exemplifed using tables or maps. The chapter concludes with a refection on the importance of the census as a key data source for small area analysis and a plea that, in a post-census world, suffcient steps be taken by central government to ensure the creation and provision of information systems for monitoring DOI: 10.4018/978-1-61520-755-8.ch001