We first explore all the Algerian statistical data collected on the informal sector, with a critical assessment upon the sources and estimation methods used. We estimate the share of the informal sector in total employment designing an equivalence table that allows an institutional divide, which fits national accounts. We also identify the characteristics of informal production units and their evolution over 2001-2010 from labour force surveys conducted by the National Statistical Office. First, we apply a one-dimensional then a multidimensional analysis, according to which the informal sector in Algeria accounts for over one third of total employment during the decade under review. These small production units, without premises or a facility specially designed for this purpose, operate in retail trade, hotels and restaurants, construction, the manufacturing industry and transport and communication.