Overview of the Tianlai project The objective of the Tianlai project is to make a 21 cm intensity mapping survey of the northern sky in the post-reionization epoch. 21cm intensity mapping is a technique for measuring the large scale structure of the Universe using the redshifted 21cm line from neutral hydrogen gas (HI). The redshifted 21cm line can be used to make three-dimensional, tomographic maps of the universe by tuning the receivers to a range of frequencies. The technique promises to allow large-volume surveys for studying the formation of the first objects during the Epoch of Reionization and for providing constraints on inflation and dark energy models. By observing the fluctuations in the large-scale distribution of neutral hydrogen, 21cm intensity mapping can provide an accurate measurement of the baryon acoustic oscillation (BAO) scale and power spectrum. The Tianlai project aims to test the technology for making 21 cm intensity mapping observations with an interferometer array. Tianlai consists of two arrays, one consisting of dish antennas and the other of cylinder reflector antennas, both located at a radio quiet site in Xinjiang, China. Both can be tuned to observe in a 100 MHz - wide band between 400 MHz and 1430 MHz, corresponding to 2.55 < z < −0.01.