The dependence of flow stress of body centered cubic metals on variables such as strain rate, temperature, strain and microstructural is a research area of continued interest. Recently, there has been renewed interest in deformation of fine grained BCC metals, which display characteristics that are different from their coarse-grained counterparts. Deformation mechanisms, strain-rate and temperature dependence, and strain hardening characteristics of fine-grained BCC metals are not well understood. The aim of this thesis is to understand the effect of strain-rate, temperature, strain and microstructure (i.e., grain size) on the mechanical response of poly¬crystalline tantalum. Among the topics addressed were constitutive modeling of flow stress, understanding the microstructural origins of strain hardening, and characterizing the effect of severe plastic deformation (SPD) on microstructure and mechanical properties. Rolling and equal-channel angular pressing (ECAP) were among the processing techniques employed. Mechanical testing was conducted over a range of temperatures and strain rates, and this was supported by a slew of microscopic characterization methods. It was found that the strain hardening response depends on microstructural evolution at different strain rates. Results indicate that the same thermally activated mechanisms operate in both as-received and processed material and this was found to be the overcoming of Peierls barriers via a double-kink mechanism. Lastly, it was found that the low strain rate sensitivity of SPD processed material was not due to fine grain size, but instead due to high internals stresses.