I have a requirement to filter a list of Clients based on if they haven't had any jobs booked in the last x months. In my code I have two lists, one is my Clients and the other is a filtered List of Jobs between today and x months ago and the idea is to filter Clients based on their id not appearing in the jobs list. I tried the following: filteredClients.Where(n => jobsToSearch.Count(j => j.Client == n.ClientID) == 0).ToList(); But I seem to get ALL clients regardless. I can easily do a foreach but this severly slows down the process. How can I filter the client list based on the job list effectively?