Key content The safety of diagnostic imaging in pregnancy is a real concern. In general, X‐rays and computed tomography scans are unlikely to cause deterministic effects in an individual pregnancy. Apart from the adverse effect of iodine on the fetal thyroid gland, common nuclear medicine investigation procedures are unlikely to involve teratogenic fetal doses. Iodinated agents should be avoided in pregnancy. It is recommended that paramagnetic contrast agents used with magnetic resonance imaging are only considered in pregnancy if absolutely necessary. Learning objectives To understand the principles, diagnostic value and safety of various diagnostic modalities using ionised radiation, at different stages of pregnancy. To choose the most appropriate imaging tool for various medical conditions in pregnancy. To be able to counsel pregnant women and provide them with accurate information about the safety of various diagnostic imaging modalities. Ethical issues Physicians should carefully weigh the risks and benefits of any radiographic procedure and include the mother in the decision‐making process whenever possible. Please cite this article as: Eskandar OS, Eckford SD, Watkinson T. Safety of diagnostic imaging in pregnancy. Part 1: X‐ray, nuclear medicine investigations, computed tomography and contrast media. The Obstetrician & Gynaecologist 2010;12:71–78.