Starburst galaxies have been the evident targets for the first molecular line surveys given the strong molecular emission. Moreover, these galaxies are indeed the most prominent molecular emitters outside the galaxy. The study by Wang et al. (2004) towards the central region of NGC 4945 is likely the first systematic detailed study of the chemical composition in an external galaxy. Using the Swedish-ESO Submillimeter Telescope (SEST), they observed 80 transitions from 19 different species over a frequecy range between 82 and 354 GHz. The derived molecular abundances and isotopic ratios measured in NGC 4945 were compared to those available in the literature from M 82 and NGC 253. Though quite complete, this comparison was limited by the selection of the observed molecular species. Line surveys over wide frequency ranges, on the other hand, provide a complete and unbiased description of the chemical composition in galactic nuclei. The first unbiased spectral line survey was carried out with the IRAM 30m telescope towards the central region of the starburst galaxy NGC 253 and covered the 2 mm atmospheric window between 129 and 175 GHz (Martín et al. 2006). This scan resulted in the detection of 111 spectral features from 25 molecular species as well as three hydrogen recombination lines. The follow up towards M 82, the brightest molecular emitter together with NGC 253, surveyed the same 2 mm window as well as the 1.2 mm band between 241 and 260 GHz (Aladro et al. Submitted). Although expected from previous observations, the detection rate in M 82 was significantly lower, with 72 spectral features from 18 molecular species. The combination of both line surveys resulted in the most complete comparison between the molecular composition of two extragalactic sources. Indeed, the observed chemical differences are claimed to be directly linked to the different state of evolution of their nuclear starbursts, and therefore to the different leading heating mechanisms affecting the ISM within their central few hundred parsecs. While a number of species, such as CH3OH, HNCO, NH3, SiO, NS, HOCO, and CH2NH,claimed to be formed in dust grains and injected into gas phase via shocks, were found to be enhanced in NGC 253, other molecules, such as CO+, HCO, HOC+, c-C3H2, CH3CCH, and NH2CN, mostly formed in the gas phase and likely enhanced by intense UV fields, are significantly more abundant towards M 82. Even though some of these differences were previously known, a number of them (in italics in the list above) were pointed out from the observations covered by the line surveys. In particular, line surveys allow the identification of particularly interesting and contrasted species such as the case of methanimine (NH2CN) which shows rotational temperatures similar to those measured towards the Galactic hot core Sgr B2(M) in both M 82 and NGC 253, but shows an abundance enhancement towards the former.