Abstract: Focal ablation surgery, an alternative to resection that seeks targeted destruction of unwanted tissue, can benefit greatly from intraoperative diagnostic imaging whenever the extents, or effects, of treatment are not directly visible or predictable. MRI, sonography and computed tomography currently dominate this field, yet important unmet medical needs still exist wherever these techniques are ill-suited for the surgical technique in question for reasons of cost, convenience or simply an inability to accurately detect changes indicative of treatment in a timely manner. Electrical impedance tomography (EIT) is a developing technique that seeks to recover images of electrical impedance from measurements made at electrodes. Importantly, it provides a new way to look at the body, and also promises to allow inexpensive real-time imaging. However, bioimpedance results from the contributions of many different phenomena acting in parallel, making interpretation of impedance images difficult. The present work seeks to address this by identifying relevant indicators. One well-posed application of EIT is for monitoring of cryosurgery, a technique which seeks to ablate tissues via freezing. To this end, 3D impedance images of the ice front were reconstructed for a cryosurgical trial in an agar gel model as well as in an in vivo rat model. Further, since low frequency impedance of tissue results primarily from ionic conduction, and cell membranes represent a strong barrier to these currents, changes in membrane permeability can be imaged with EIT, allowing detection of cell viability. Impedance images during thawing show that 1 kHz conducitivity increases to 2-2.5 times its value before cryosurgery, which matches closely with ex vivo experiments conducted under various freezing protocols. First in vivo trials are then described for irreversible electroporation, a new focal tissue ablation technique, showing promise for nonthermal treatment of inoperable tumors. The effects on tissue impedance are then explored, culminating with the recovery of impedance images from a non-trivial pulse geometry in an in vivo model. There are many applications for monitoring tissue state, for instance in organ transplantation, and this dissertation shows that electrical impedance tomography is well-suited for these purposes whenever the consistent changes in impedance are sufficiently well understood.