In response to new customer requirements, market dynamics, mergers, and technological innovation, modern software development organizations are adopting agile software development (ASD). Yet, the simple adoption of agile methods such as Scrum or eXtreme programming does not automatically result in a very agile team. While we understand the introduction and adoption of ASD from a methodical perspective, we have yet to explore design principles that guide methodical extensions of ASD, and we need to learn more about the human factors that influence software development teams. This thesis presents four studies. Studies 1 and 2 investigate the methodical extension of ASD by identifying design principles from secondary data. Study 1 extends ASD with processes and practices from user-centered design. Study 2 investigates early activities that precede development activities. The thesis also investigates human factors of agile software development in studies 3 and 4. Study 3 compares teams along their extents of agility in order to identify influential factors using a multicase study design. Study 4 tests the effects of emotional contagion ... mehrin virtual software development teams using a large dataset from an open source software repository. Thus, this thesis makes two primary contributions. First, it develops design principles for methodical extensions of ASD; second, it contributes to the human factors that influence software development teams. Managers also receive guidance on the improvement of ASD in their organization.