Constructivist theory is an important underpinning concept when thinking about using or creating a WebQuest.

(Source: Constructivist Theory)
An important part of Constructivism is that the student will construct meaning especially if they are given the chance within a group to talk about their ideas. An excellent paper by Prof. George E. Hein, Lesley College. Massachusetts USA, [The Museum and the Needs of People] explains the Principles of Learning and lists the following:
"1. Learning is an active process in which the learner uses sensory input and constructs meaning out of it
2. People learn to learn as they learn: learning consists both of constructing meaning and constructing systems of meaning.
3. The crucial action of constructing meaning is mental: it happens in the mind. [Although with young children and even adults, hands-on activities help the mind].
4. Learning involves language: the language we use influences learning.
5. Learning is a social activity [
6. Learning is contextual [in a WebQuest, the problem has to be authentic]
7. One needs knowledge to learn: it is not possible to assimilate new knowledge without having some structure developed from previous knowledge to build on. The more we know, the more we can learn.
8. It takes time to learn: learning is not instantaneous.
9. Motivation is a key component in learning." (Constructivist Learning Theory)
ALL these elements are important in a WebQuest!
Sites:
Constructivist Learning Theory by Prof. George E. Hein
Constructivist Theory
Constructivist Theory
Constructivism
University of Adelaide: Constructivist Theory of Learning