Hack and slash tutorial ModifiedStat Tutorial

I've got the error:

The type or namespace name List' could not be found. Are you missing a using directive or an assembly reference? And a warning: >The member ModifiedStat.AdjustBaseValue' does not hide an inherited member. The new keyword is not required

Code:

 using System.Collections.Generic;
 
 public class ModifiedStat : BaseStat {
 private List _mods; //A list of Attribute that modify the stat 
 private int _modValue;                     //Amount added to the baseValue from the modifiers
 
 public ModifiedStat(){
 _mods = new List();
 _modValue = 0;
 }
 public void AddModifier( ModifyingAttribute mod){
 _mods.Add(mod);
 }
 
 private void CalculateModValue(){
 _modValue = 0;
 
 if(_mods.Count > 0)
 foreach(ModifyingAttribute att in _mods)
 _modValue += (int)(att.attribute.AdjustBaseValue * att.ratio);
 
 }
 
 public new int AdjustBaseValue{
 get { return BaseValue + BuffValue + _modValue; }
 }
 
 public void Update(){
 CalculateModValue();
 }
 }
 
 public struct ModifyingAttribute{
 public Attribute attribute;
 public float ratio;
 
 public ModifyingAttribute(Attribute att, float rat){
 attribute = att;
 ratio = rat;
 }
 }

It doesn't like

 private List _mods;

However if I put

 private List<ModifyingAttribute> _mods;

It comes up with even more errors!