We are well aware that organisms exhibit different states of awareness, from sleep to heightened tension. Cells can likewise be in different states (phases) where different response networks and different functions are active. The different phases of the cell cycle are obvious examples of how the same cell can exhibit different behaviors when stimulated in a defined way without changing its differentiation state. Furthermore, a given function may have different activators and different levels of activity in different phases. Thus, it is important to control the cell phase when studying any given function. With a large population of cells in a variety of cell phases, the assay of a function will potentially give a mixture of activity levels. There are many different possible phases for cells, including changes in type of motility or metabolic activity in addition to the well-characterized phases of the cell cycle, senescence, or apoptosis. It is important to consider how the cell decides to transition from one phase to another. As was discussed in the previous chapter, the inability of a cell to complete a desired function can be a stimulus to change to an alternative pathway that can involve a change in phase. Further, phase changes can also be induced by the internal cell clock, or external stimuli. Another issue is that widely different cell types can exhibit very similar phases (e.g. mitosis is similar for different types of cells). Thus, many of the characteristic functions associated with different phases are present in many cell types but need specialized conditions to become activated. The same function may have varied outputs in different cell types, but the basic elements would be common to all cells. Often the transitions between phases occur in a very short time and constitute a concerted change. The probability that cells in a population will be in the same phase can be controlled in the lab using patterned substrates and defined conditions. However, single-cell studies can provide a better understanding of the cell phase when a given subcellular function is analyzed. Using defined molecular markers for specific phases, it is possible to follow phase changes at a single-cell level in multicellular tissues.