The aim of this article is to present a method of assignment of defence lawyers to causes, using tools Petri nets, decision trees and Monte Carlo method. It presents a simulation model of assignment applied to law activities, specifically to defender lawyers contracted by tender, using Petri Nets, Montecarlo and decision trees. The main difficulty of this assignment task is how to distribute equally the workload along the year between two kinds of lawyers: defender lawyers' staff and external (tendered) defender lawyer. The system utilized for analyzing task assignment is stochastic, parallel, concurrent and dynamic. (C) 2016 The Authors. Published by Elsevier B.V.