problem with undefined reference

Hi,
    Iam facing problem with undefined reference linking errors.
To explain in more detail..
I have a class called TOP. In its constructor I have instantiated many
other class objects using pointers.
Iam getting errors like
1. In function TOP::TOP(int) : undefined reference to
CHILD1::CHILD1(int, float).
2. In function TOP::TOP(int) : undefined reference to vtt for
CHILD2::CHILD2(char*)
3. In function TOP::TOP(int) : undefined reference to vtable for
CHILD3::CHILD3( )
4.In function TOP ::func() undefined reference to CHILD3:: func2()

What could be possible reasons for getting such errors.