Our life and our projects are part of complex systems where variables are often interconnected. Complexity is the science that analyzes the phenomena that emerge from a collection of numerous elements interacting in numerous forms within a system. The behavior of complex systems such as the human brain, Earth climate, socio-economic organizations (ie. cities, businesses), are difficult to model due to the interdependencies, relationships, and interactions between their parts and with its particular environment. They have distinct properties that arise from these relationships, such as nonlinearity, emergence, spontaneous order, adaptation and others.
Complexity can be distinguished as disorganized complexity, and organized complexity. In disorganized complex systems interactions can be seen as largely random, the properties of the system as a whole can be understood by using probability and statistical methods. Organized complex systems involves non-random, or correlated, interaction between the parts. These correlated relationships create a differentiated structure that can, as a system, interact with other systems. The coordinated system manifests properties not carried or dictated by individual parts, it can be said to "emerge," without any "guiding hand".