The long term goal of this work is to harness the effect of bevel tip needle bending to provide accurate, dexterous targeting for percutaneous therapies. New results in needle and tissue modeling, combined with robot motion modeling, will facilitate new models, hardware, control, and planning techniques for steering flexible needles inside soft tissue. Using path planning, control, and kinematics results from the field of robotics, we plan to demonstrate that an appropriately designed needle can be steered through tissue to reach a specified 3-D target. Even more compelling is that the methods we propose will allow needles to be steered to previously inaccessible locations in the body, enabling new minimally invasive procedures. The first step in needle control is to obtain an accurate model. Thus, the specific goals of this exploratory/developmental application are to under stand and model the biomechanics of needle motion. This is a high-risk activity because there has been no previous work to successfully model the 3-D path of flexible needles through soft tissue. The work is high impact because accurate 3-D needle modeling can be used to (1) improve targeting, thus enhancing the performance of many percutaneous therapies and diagnostic methods, (2) plan needle paths that steer around obstacles, and (3) develop realistic simulators for physician training and patient-specific planning. Working closely with physician collaborators, the investigators will study needle insertion scenarios that are relevant to improving the quality of health care. The specific aims are as follows: (1) Develop and validate a deterministic biomechanical model of needle insertion through un deformed tissue. The bending of the need le tip will be a function of tissue properties, needle properties and input parameters. Refine and test the model on phantom tissues. Select needle design parameters that facilitate steering. (2) Further refine the model of Aim 1 to include the effects of constraint and input uncertainties, generating a stochastic biomechanical model of needle insertion through undeformed tissue. Compute the space of possible needle tip positions for a given set of inputs and determine the probability of acquiring specific targets. (3) Further refine the models of Aims 1 and 2 to include the effects of both tissue deformation and inhomogeneous tissue properties. A particular challenge is the development of new modeling techniques that simultaneously handle needle and tissue deformation.