In this Thesis, first, Shape Memory Alloys (SMAs), whose application is growing in industries, are introduced. SMAs show thermo-mechanical couplings in their responses so their mechanical properties can change due to change in temperature. This phenomenon originates from stress- or temperature-induced martensitic transformation. These particular materials have two specific responses of Pseudoelasticity (PE) and Shape Memory Effect (SME). Above the austenite temperature, these materials show hysteresis in their responses accompanied by a large recoverable strain. In addition, SMAs can recover their imposed strain below the austenite temperature once heated. Due to these two characteristics, researchers have found SMAs suitable for many areas including control of smart systems. Since SMAs do not follow the typical normality rule of usual metals, especially under nonproportional loadings, modeling these alloys under 3-D loading paths is challenging. For instance, normality rule cannot be used for SMAs due to martensite reorientation. Another challenge modeling SMAs is tension-torsion coupling which means production of axial strain in shear. One of the efficient methods in 3-D modeling for SMAs is Microplane theory. In this approach, by applying 1-D constitutive equation on any plane passing through any material point, once can obtain a 3-D constitutive equation. Although this theory has many effective features compared the other methods, it cannot predict tension-torsion coupling using the current numerical methods. The purpose of this thesis is to employ Microplane theory and its predictions under proportional and nonproportional loadings using the 21-point numerical method of Ba?ant and to evaluate the resultant errors. To add tension-torsion coupling to Microplane theory for resolving the numerical errors, many different methods have been studied. Throughout this study, a 25-point numerical method shows acceptable results, by applying which, Microplane theory would has the ability to predict tension-torsion coupling. The numerical results are compared with experimental findings, and good agreements are shown to exist. Keywords: Shape memory alloys, Microplane modeling, Proportional Loading, Nonproportional Loading, Tension-Torsion Coupling