Talmi's main field of research is the theory of nuclear structure. The atomic nucleus can be composed of a large number of protons and neutrons which move due to strong interactions between them. In spite of their complexity, nuclei exhibit some simple and regular features. Most importantly, nuclei behave as if they move independently in a common static potential well. This gives rise to the existence of shells of protons and neutrons much like the electronic shells in atoms. Nuclei whose proton and neutron shells are complete have special stability and the numbers of protons and of neutrons in them are called magic numbers. This picture of the nucleus is called the nuclear shell model to obtain the information from experimental data and use it to calculate and predict energies which have not been measured. This method has been successfully used by many nuclear physicists and has led to deeper understanding of nuclear structure. To calculate energies of nuclear states it is necessary to know the exact form of the forces which act between the nuclear constituents. These are still not sufficiently known even after many years of research. Talmi developed a method to obtain the information from experimental data and use it to calculate and predict energies which have not been measured. This method has been successfully used by many nuclear physicists and has led to deeper understanding of nuclear structure. The theory which gives a good description of these properties was developed. This description turned out to furnish the shell model basis of the elegant and successful interacting boson models. Talmi also participated in the study of explicit fermion–boson mappings required to connect the interacting-boson model with its shell-model roots and in the introduction of the boson F-spin analog to nucleon isospin.