As shown in Chapter 1, most of our current knowledge about migration in the past comes from census birthplace data, supplemented by sources such as Poor Law certificates, apprenticeship registers, diaries and oral reminiscences. Having reviewed the current state of knowledge about migration in the past derived from such sources, this chapter focuses in detail on the strengths and limitations of different classes of data for the analysis of migration. It should be stressed that this study is largely dependent on the same sorts of sources that other researchers have used, and thus the limitations of such sources need to be fully appreciated. However, the range of material utilized and the methods used to collect individual longitudinal data in this study overcome some of the previous difficulties. These methods, and the characteristics and biases of the data produced, are explained in the second half of this chapter.