LTL is a C++ class template library for scientific computing which provides high performance via SSE3 floating point support for vector operations. Useful for array processing, image processing, FITS and ASCII I/O, and linear algebra (astronomical and scientific computing, in short). LTL provides dynamic arrays of up to 5-dimensions, subarrays and slicing, support for fixed size vectors and matrices including basic linear algebra operations, expression templates based evaluation, and I/O facilities for ascii and FITS format files. Users of the boost and blitz++ library may find the cross-pollination of these unique features to be fruitful. WWW: http://www.mpe.mpg.de/~drory/ltl/