SENSMG is a tool for computing first-order sensitivities of neutron reaction rates, reaction-rate ratios, leakage, keff, α, and subcritical multiplication using the PARTISN multigroup discrete-ordinates code. 1 SENSMG computes sensitivities to all of the transport cross sections and data (total, fission, a nu, chi, and all scattering moments), four edit cross sections (absorption, capture, elastic scattering, and inelastic scattering), and the density for every isotope and energy group. It also computes sensitivities to the mass density for every material and derivatives with respect to all interface locations and outer boundaries. The tool can be used for one-dimensional spherical and slab (r) and two-dimensional cylindrical (r-z) geometries. The tool can be used for fixed-source and eigenvalue problems. For most responses, the tool implements Generalized Perturbation Theory (GPT) as discussed by Williams 2 and Stacey. 3 The tool is thus limited to computing sensitivities only for GPT-allowable responses. 4 For subcritical multiplication, the tool implements sensitivities derived by O'Brien 5 and Clark. 6 SENSMG has a similar role as the old SWANLAKE (Ref. 7), FORSS (Ref. 8), and SENSIT (Ref. 9) codes. It has capabilities similar to those of SUSD3D (Ref. 10), which also uses PARTISN. Section II of this report describes the theory behind adjoint-based sensitivities, gives the equations that SENSMG solves, and defines the sensitivities that are output. Section III describes the user interface, including the input file and command line options. Section IV describes the output. Section V gives some notes about the coding that may be of interest. Section VI presents some sample problems and discusses verification, which is ongoing. Section VII lists needs and ideas for future work. Appendix A lists most of the input files whose results are presented in Sec. VI. Appendix B provides some useful details on one of the cross-section libraries that SENSMG supports. a Technically, fission is an edit cross section in PARTISN because only the product with nu is used in the transport.