HI Cosmology at $z = 0:$ a Brief Review

Extragalactic HI astronomy is half century old. Its maturity dramatically increased in the 1970s, with the commissioning of new powerful facilities. Its contributions to Cosmology are important, from the observation of galaxy rotation curves that showed the presence of dark matter in galaxies, to the measurement of cosmological parameters and the mapping of the large scale structure of the Universe. The Arecibo telecope has played a key role in these developments. It is also currently engaged in a number of experiments that utilize its L‐band feed array to map thousands of square degrees of the sky and obtain the most sensitive large‐scale view of the low z HI Universe.