UNCSAM 1.1: A Software Package for Sensitivity and Uncertainty Analysis. Sensitivity and uncertainty analysis are generally recognized as important and indispensable parts of the modelling process. The easy availability of tools for performing these analyses contributes substantially to a reliable and efficient development, assessment and application of mathematical models. For this reason a comprehensive and flexible software package UNCSAM has been developed at the RIVM for performing sensitivity and uncertainty analysis on a large variety of models. UNCSAM combines a Monte Carlo based approach (sampling and simulation) with regression- and correlation analysis, to obtain a clear picture of the sensitivities and uncertainties and their main contributors. The package is developed in ANSI standard Fortran 77, embedded in an ANSI-C shell for dynamic memory allocation, and can be installed on a variety of devices. The manual describes the scope, installation and use of UNSCAM, release version 1.1.