The value of the modern cancer registry and its ability to carry out cancer control activities rely heavily on the underlying quality of its data and the quality control procedures in place. This two-part review provides an update of the practical aspects and techniques for addressing data quality at the cancer registry. This first installment of the review examines the factors influencing three of the four key aspects, namely, the comparability, validity and timeliness of cancer registry data. Comparability of cancer data may be established through a comprehensive review of the registration routines in place. Validity is examined via numerical indices of that permit comparisons with other registries, or, within a registry, over time, or with respect to specified subsets of cases. There are no international guidelines for timeliness at present, although specific standards for the abstraction and reporting of registry have been set out by certain organisations.