Constraints are limitations that restrict our freedom of action in some way. Limitations in the time and resources available for some activity are age-old constraints that have affected almost every human endeavour since the dawn of civilisation. Every project we undertake must usually be completed by an agreed date with limited resources, and there will only be a certain amount of money available to fund the work. The imposition of a fixed timescale and a budget on any undertaking forces us to consider the implications of falling behind with the work or spending too much money. There are also the constraints imposed due to the need to carry out certain tasks in a specific order. If we were building a computer network, for example, we would need to acquire and install network accommodation (i.e. equipment racks and cabinets) before installing the major items of networking equipment. Any task that cannot be started until one or more other tasks have been completed is said to have a task dependency. One of the most important goals of project management, therefore, is to ensure that we are aware of the constraints that exist, and tailor our plans accordingly.

Having planned work schedules and expenditure around known constraints, we next have to consider the possibility that further constraints may be imposed on us by unforeseen circumstances at some point during the implementation phase of the project. It may be the case that there are a number of tasks that do not have any task dependencies, and may be performed at the same time (i.e. in parallel). This implies that at certain times during the execution phase of the project there will be a high level of activity, while at other times there may be relatively little happening. This in turn implies that during certain periods there will be a high demand for resources, particularly manpower, in order to get the work done on schedule. Suddenly finding that key members of the project team are unavailable during such a period due to illness or having been seconded to another project may force you to postpone one or more activity, which can have a negative impact on the project's target completion date. Other constraints can be imposed by delays such as those caused by suppliers not being able to deliver equipment or materials on time, which also has the potential to delay project completion.

One of the aims of the planning process should be to smooth out any peaks in demand for resources wherever possible, which usually means trying to spread the workload evenly throughout the project's lifetime. Where delays are inevitable, one option for bringing the project schedule back on track is to allocate more resources (usually in the form of manpower) to those tasks that are significantly delaying the project. This may marginally increase overall costs (where, for example, overtime has to be paid) but may be preferable to extending project deadlines, especially where time is at a premium (in which case the project is said to be time-constrained). Most projects incur costs in the form fixed overheads during the project’s implementation phase. The project overheads are normally proportional to the amount of time taken to complete the project, so any delay is likely to result in additional cost, regardless of whether or not work is taking place. On the other hand, in situations where resources are at a premium (in which case the project is said to be resource-constrained) and overheads are at a relatively low level, it may be better to plan for a somewhat longer timescale in order to smooth out the demand for scarce resources.