A web-based application for alignment visualization, browsing and analysis of genome sequence data. LookSeq supports multiple sequencing technologies, alignment sources, and viewing modes; low or high-depth read pileups; and easy visualization of putative single nucleotide and structural variation. The visible range, from whole chromosome to single base resolution, can be set manually or by scrolling or zooming the display with fast, on-the-fly rendering from the server-side alignment database. LookSeq uses a universal database for alignments of different sequencing technologies and algorithms. Sequence data from multiple sources can be viewed separately or aligned in a single display, facilitating direct comparison between datasets. LookSeq can also link to relevant external sites such as PubMed and other online analysis tools, via buttons or double-clicking on the displayed sequence annotation.