In this dissertation, I argue that visual composition tools should reify visual properties— that is, create them into first-class, interactive objects that designers can manipulate, directly in the document. Artists and designers use visual properties such as color, typography,size, and position to create novel composition concepts. Most visual composition tools treat graphical elements as objects— but not their visual properties. The latter are simply attributes of graphical elements, usually accessible on demand, through property sheets or dialog boxes, rather than as independen tinteractive objects.I begin with an introduction, where I outline the dissertation and summarise the main contributions.Then, I present a conceptual overview, where I argue that perception, expertise, theories and existing representations of visual properties affect how designers manipulate them in their work. In the related worksection, I identify the main approaches visual composition tools propose to manipulate visual properties.I explain the advantages and limitations of these approaches.I present a comparative structured observation study (Chapter 4) where graphic designers perform visual composition tasks in Adobe Illustrator. Designers preferred tools that provide direct access to visual properties.They use these visual properties to complete visual composition tasks such as alignment and distribution.I then present two interview studies (Chapter5, Chapter 6) that examined artists’ and designers’practices as they manipulate color and create layouts in their projects. Artists and designers create personal color representations. They manipulate each color in the context of its surrounding graphical elements,and combine it with other visual properties such as texture. As they create their layouts, designers establish links among visual properties such as size,position, and layering of graphical elements. They define rules for how these properties change in space,across instances of the same composition, or in time,across related compositions.Based on these observations, two groups of prototypes (Chapter 7) demonstrate how we can reify visual properties into first-class graphical objects. ColorPartner supports creating personal representations of the color manipulation process. Color Revealer permits creating informative color representations of how a process evolves. Palette Explorer supports manipulatingvisual properties of color swatches, such as size, position and layering, in the context of surrounding colors. Color Compositor supports manipulating the combination of colors and textures. Linkify supports linking visual properties directly in the documentand defining rules for how these links evolve as the content changes. Contextify supports creating rules that define how visual properties change across instances of a composition. In two follow up studies,designers explain how they intend to use these tools in their current projects..I conclude with a summary of the contributions(Chapter 8) and a discussions of the limitations and future work.