Preliminary results of space-time radon monitoring in the Northern Tien Shan are presented. The study area was chosen on the basis of the known seismic events. A basic model of the observed effects is determined. The radon exhalation is shown to depend nonlinearly on variations in the stress state of the medium and the recording mode of the radon field. A geodynamic model of earthquake nucleation is presented and the expected effects in the radon field are examined. Results of measuring radon field variations before earthquakes are presented. It is shown that noticeable and characteristic changes in the radon field are observed, respectively, 90–100 and 3–5 days before a seismic event. Constraints are gained on the response of the radon field within a sedimentary rock mass to the passage of waves generated by distant earthquakes. The data presented in the paper show that radon monitoring can be successfully used for studying the process of earthquake nucleation. The model of tectonic earthquake nucleation proposed in the paper uses data of multidisciplinary geophysical monitoring (including radon monitoring) to obtain a deterministic indicator of earthquake nucleation.