So, gangs vary in relation to one another across and within communities, and the individual street gang varies within itself. All of this variation in group properties makes the street gang hard to define, measure, and respond to. Street gangs exhibit varying degrees of structure, organization, and cohesion. They aren't all tightly knit or highly structured by any means. Some are very cohesive criminal enterprises with clear and strong leadership at the top, while most are loose confederations of youth and adults whose associations grow out of the shared troubles of their respective biographies.
Street gangs are hard to define for other reasons, too.
As porous social groups, gangs experience tremendous turnover of members. Youth come and go, often staying involved for about a year before moving on. Gang leadership also changes over time. A leader today may be gone tomorrow because of a family move, voluntary disaffiliation, or incarceration. Another challenge to defining the street gang is the ambiguity surrounding gang membership.