Most 'flat' babies with acidosis will be septic, some might have another condition, but occasionally we will have the opportunity to diagnose and treat a congenital metabolic condition. Individually these conditions are rare, but as a group they are collectively common enough that we will encounter them in our Emergency Departments. Classifying these conditions is an ever-changing minefield, but fortunately, understanding the basic principles is not too bad... especially with a bit of expert guidance from our guest brain.