To ensure high availability and fault tolerance with Elastic Load Balancers (ELB) in Amazon Web Services (AWS), employ a multi-Availability Zone (AZ) architecture. Register instances in multiple AZs with your ELB to distribute incoming traffic. If one AZ fails or becomes overloaded, ELB automatically reroutes traffic to instances in the remaining healthy AZs. Use ELB's health checks to send traffic only to healthy instances. For high availability, enable cross-zone load balancing to distribute traffic evenly across all registered instances in all enabled AZs. You can also use Auto Scaling Groups with ELB, ensuring the dynamic adjustment of your compute capacity in response to traffic patterns and maintaining high availability even during demand spikes.