This paper is founded on a tutorial session given to the School on Modern Statistical Methods in Medical Research which was held at the International Centre for Theoretical Physics, Trieste in September 1999. We review the aims, scope and purposes of infectious disease surveillance including determining transmission information to underpin model structure and parameterization in epidemic modelling. The practical problems inherent in collecting surveillance data are illustrated by a study of HIV/AIDS in Cambodia. We also review the basic elements of mathematical models developed to represent the transmission dynamics of infectious diseases, and discuss reasons for the gap between mathematical epidemic models and available data.