This section discusses the evolution of a diffusing cloud once it encounters a boundary. Emphasis is placed on two common types of boundaries, namely those that are perfectly absorbing and those through which there is no flux. The theory section describes how the use of image sources allows us to correctly model the concentration profile of the cloud in the presence of a boundary. The animation compares the effect that no-flux and perfectly absorbing boundaries have on a diffusing cloud, highlighting the progression to a well-mixed condition in the presence of parallel no-flux boundaries.