What is a testamentary trust?

What is a testamentary trust? Testamentary trusts are simply trusts created in a Will.

Usually the term is used to refer to a "discretionary" trust set up within a Will, which have classes of beneficiaries (usually down a beneficiary's bloodline). The trusts can have substantial tax and asset protection advantages over directly gifting an inheritance.

But there must be a proper evaluation of whether the trusts are necessary for a will-maker's particular circumstances and those of their family. 
Principal Solicitor Richard Morris briefly discusses testamentary trusts in this clip