We present eXtended CASA Line Analysis Software Suite (XCLASS) a toolbox for the Common Astronomy Software Applications package (CASA) containing a couple of new functions for modelling interferometric and single dish data. The toolbox contains an interface for the model optimizer package MAGIX (Modeling and Analysis Generic Interface for eXternal numerical codes), which helps to find the best description of the data using a certain model, i.e., finding the parameter set that most closely reproduces the data. Additionally, the toolbox provides a function to model the data by using the myXCLASS program. The myXCLASS program models the data by solving the radiative transfer equation for an isothermal object in one dimension, whereas the finite source size and dust attenuation are considered as well. Furthermore, the toolbox contains an interface for VAMDC (Virtual Atomic and Molecular Data Centre) and the CDMS (Cologne Database for Molecular Spectroscopy) database.