Natural cements calcined at low temperatures, so-called Roman cements, formed an important binder material in 19th century construction and facade decoration of many European cities and towns. As a rule, Roman cement mortars appear well preserved. In order to understand their range of composition and properties, a number of samples from historic buildings were collected and analyzed. Microscopic techniques including scanning electron microscopy were employed along with the assessment of physicomechanical properties. The study shows that the most significant feature of Roman cement mortars is a wide range of differently calcined clinker relicts within each mortar. These unreactive particles, classified as being either overfired or underfired, obviously play an important role for the mortar properties. The binder relicts are composed of a number of phases in the system Ca-Si-Al-Fe. C₂S and C₂AS (gehlenite) are among the most frequent compounds, their grain size and microstructure depending on calcination temperatures. In particular, the clinkers produced at lower temperatures show nonequilibrium features such as solid solution systems and zoning by partial diffusion. The aggregates found in the mortars cover a wide range of mineralogical compositions reflecting local geological conditions. Cast and in situ applied mortars differ in the amount of aggregate, which is generally lower for cast elements. There is no correlation between the amount of inert material and the state of preservation indicated, e.g., by the occurrence of shrinkage cracks. Historic cast mortars show high compressive strengths at comparably low moduli of elasticity. The total porosity is frequently high. This contribution presents the above-mentioned properties and discusses them in terms of the excellent aging performance of the historic Roman cement mortars.