Lyapunov Exponents Toolbox (LET) provides a graphical user interface for users to determine the full sets of Lyapunov exponents and Lyapunov dimension of continuous and discrete chaotic systems. This toolbox can only run on MATLAB 5 or higher versions of MATLAB. It has been tested under Windows and Unix and may also run on other platforms. To run the program, enter LET in MATLAB command window. Detail description of this toolbox can be found in the file README.M.