This chapter summarizes some of the most important aspects of current knowledge in relation to clinical assessment of placentation. Ultrasound allows the assessment of some physical characteristics of the placenta, such as thickness and, more recently using 3D methods, volume. Uterine artery Doppler flow velocimetry can be used to assess the resistance to blood flow in the uterine circulation. First-trimester maternal serum levels of pregnancy-associated plasma protein A (PAPP-A) have been widely studied in the assessment of Down's syndrome risk. Clinical manifestations of serious complications of pregnancy, such as growth restriction, pre-eclampsia and stillbirth are preceded by abnormalities in clinical biomarkers of placentation. Several large-scale prospective cohort studies are in progress which aims to improve on currently available methods for clinical detection of impaired placentation. Multiple lines of evidence indicate an important role for abnormal placentation in the pathophysiology of many of the most important complications of pregnancy.