There are many factors that will affect the appearance of concrete, but before these can be discussed intelligently, the blemishes themselves must be accurately identified and classified. This article, based on research conducted at the Wexham Springs Laboratories of the Cement and Concrete Association, London, classifies the surface defects. The term "discoloration" is used to describe all blemishes which are distinguished by variations in color or shade, irrespective of the scale of the defect. "Physical irregularities" refers to all mechanical imperfections including damage to the finished surface. Physical irregularities may, of course, be accompanied by associated discoloration. Similarly, several blemishes are known to change in character and severity from the time that they are first observed. In such a case that definition listed here applies to the earliest known form of the defect. This means that some of the commonest name for blemishes have been omitted from the list found in the article. Such omissions are deliberate, however, since most of these blemishes are now known to be either multiple defects, or different forms of the basic blemishes defined. Causes of surface irregularities can be numerous because blemishes rarely occur in isolation. Tests were conducted examining the influence of form releasers and effects of pressure and materials on concrete blemishes. Tests showed that neither the neat oils alone nor the emulsifying agents alone caused and retardation or hydration discoloration. It was concluded, therefore, that the emulsifier acts as a catalyst in promoting interference by the oil. An investigation into the pressures generated during the placing of fresh concrte showed that the range of pressures commonly encountered on site has increased considerably in recent years. Blemishes associated with excessive deformation of the formwork under load are observed more and more frequently. In addition, a number of new defects appear when concrete is forcible mobilized under high pressure. Where such pressure variations are likely to be reflected in the concrete surface it is important to design both the formwork and the concrete mix to minimize these blemishes.