diff --git a/README.md b/README.md
index 3ae3aa9..d78d6ca 100644
--- a/README.md
+++ b/README.md
@@ -16,7 +16,7 @@ Tested with Mermaid js v9.1.1
 - [x] [Class diagram](https://mermaid-js.github.io/mermaid/#/classDiagram)
 - [x] [Pie chart](https://mermaid-js.github.io/mermaid/#/pie)
 - [x] [State diagram](https://mermaid-js.github.io/mermaid/#/stateDiagram)
-- [ ] [Entity relationship](https://mermaid-js.github.io/mermaid/#/entityRelationshipDiagram) https://github.com/wowbios/FluentMermaid/issues/18
+- [x] [Entity relationship](https://mermaid-js.github.io/mermaid/#/entityRelationshipDiagram) https://github.com/wowbios/FluentMermaid/issues/18
 - [ ] [User journey](https://mermaid-js.github.io/mermaid/#/user-journey) https://github.com/wowbios/FluentMermaid/issues/19
 - [ ] [Gantt](https://mermaid-js.github.io/mermaid/#/gantt) https://github.com/wowbios/FluentMermaid/issues/20
 - [ ] [Requirement](https://mermaid-js.github.io/mermaid/#/requirementDiagram) https://github.com/wowbios/FluentMermaid/issues/21
diff --git a/src/FluentMermaid/EntityRelationship/EntityRelationshipDiagram.cs b/src/FluentMermaid/EntityRelationship/EntityRelationshipDiagram.cs
new file mode 100644
index 0000000..b379b58
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/EntityRelationshipDiagram.cs
@@ -0,0 +1,9 @@
+using FluentMermaid.EntityRelationship.Interfaces;
+using FluentMermaid.EntityRelationship.Nodes;
+
+namespace FluentMermaid.EntityRelationship;
+
+public static class EntityRelationshipDiagram
+{
+    public static IEntityRelationshipDiagram Create() => new EntityRelationshipDiagramRoot();
+}
diff --git a/src/FluentMermaid/EntityRelationship/Enums/Cardinality.cs b/src/FluentMermaid/EntityRelationship/Enums/Cardinality.cs
new file mode 100644
index 0000000..1fe07ff
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Enums/Cardinality.cs
@@ -0,0 +1,9 @@
+namespace FluentMermaid.EntityRelationship.Enums;
+
+public enum Cardinality
+{
+    ZeroOrOne,
+    OnlyOne,
+    ZeroOrMany,
+    OneOrMany
+}
diff --git a/src/FluentMermaid/EntityRelationship/Enums/RelationType.cs b/src/FluentMermaid/EntityRelationship/Enums/RelationType.cs
new file mode 100644
index 0000000..e1ab182
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Enums/RelationType.cs
@@ -0,0 +1,7 @@
+namespace FluentMermaid.EntityRelationship.Enums;
+
+public enum RelationType
+{
+    Identifying,
+    NonIdentifying
+}
diff --git a/src/FluentMermaid/EntityRelationship/Extensions/CardinalityExtensions.cs b/src/FluentMermaid/EntityRelationship/Extensions/CardinalityExtensions.cs
new file mode 100644
index 0000000..c45789c
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Extensions/CardinalityExtensions.cs
@@ -0,0 +1,16 @@
+using FluentMermaid.EntityRelationship.Enums;
+
+namespace FluentMermaid.EntityRelationship.Extensions;
+
+internal static class CardinalityExtensions
+{
+    public static string Render(this Cardinality cardinality, bool left)
+        => cardinality switch
+        {
+            Cardinality.ZeroOrOne => left ? "o|" : "|o",
+            Cardinality.OnlyOne => "||",
+            Cardinality.ZeroOrMany => left ? "}o" : "o{",
+            Cardinality.OneOrMany => left ? "}|" : "|{",
+            _ => throw new ArgumentOutOfRangeException(nameof(cardinality), cardinality, null)
+        };
+}
diff --git a/src/FluentMermaid/EntityRelationship/Extensions/RelationTypeExtensions.cs b/src/FluentMermaid/EntityRelationship/Extensions/RelationTypeExtensions.cs
new file mode 100644
index 0000000..679f962
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Extensions/RelationTypeExtensions.cs
@@ -0,0 +1,14 @@
+using FluentMermaid.EntityRelationship.Enums;
+
+namespace FluentMermaid.EntityRelationship.Extensions;
+
+internal static class RelationTypeExtensions
+{
+    public static string Render(this RelationType type)
+        => type switch
+        {
+            RelationType.Identifying => "--",
+            RelationType.NonIdentifying => "..",
+            _ => throw new ArgumentOutOfRangeException(nameof(type), type, null)
+        };
+}
diff --git a/src/FluentMermaid/EntityRelationship/Interfaces/IEntity.cs b/src/FluentMermaid/EntityRelationship/Interfaces/IEntity.cs
new file mode 100644
index 0000000..bee4c2c
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Interfaces/IEntity.cs
@@ -0,0 +1,9 @@
+using System.Text;
+
+namespace FluentMermaid.EntityRelationship.Interfaces;
+
+public interface IEntity : IRenderTo<StringBuilder>
+{
+    string Name { get; }
+    IField AddField(string type, string name, string? modifier);
+}
diff --git a/src/FluentMermaid/EntityRelationship/Interfaces/IEntityRelationshipDiagram.cs b/src/FluentMermaid/EntityRelationship/Interfaces/IEntityRelationshipDiagram.cs
new file mode 100644
index 0000000..fcda8b2
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Interfaces/IEntityRelationshipDiagram.cs
@@ -0,0 +1,10 @@
+using FluentMermaid.EntityRelationship.Enums;
+
+namespace FluentMermaid.EntityRelationship.Interfaces;
+
+public interface IEntityRelationshipDiagram
+{
+    IEntity AddEntity(string name);
+    IRelation Relation(IEntity from, IEntity to, Cardinality fromCardinality, Cardinality toCardinality, RelationType relationType, string? label);
+    string Render();
+}
diff --git a/src/FluentMermaid/EntityRelationship/Interfaces/IField.cs b/src/FluentMermaid/EntityRelationship/Interfaces/IField.cs
new file mode 100644
index 0000000..71ace11
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Interfaces/IField.cs
@@ -0,0 +1,10 @@
+using System.Text;
+
+namespace FluentMermaid.EntityRelationship.Interfaces;
+
+public interface IField : IRenderTo<StringBuilder>
+{
+    string Type { get; }
+    string Name { get; }
+    string? Modifier { get; }
+}
diff --git a/src/FluentMermaid/EntityRelationship/Interfaces/IRelation.cs b/src/FluentMermaid/EntityRelationship/Interfaces/IRelation.cs
new file mode 100644
index 0000000..4e70d7a
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Interfaces/IRelation.cs
@@ -0,0 +1,14 @@
+using System.Text;
+using FluentMermaid.EntityRelationship.Enums;
+
+namespace FluentMermaid.EntityRelationship.Interfaces;
+
+public interface IRelation : IRenderTo<StringBuilder>
+{
+    IEntity From { get; }
+    IEntity To { get; }
+    Cardinality FromCardinality { get; }
+    Cardinality ToCardinality { get; }
+    RelationType RelationType { get; }
+    string? Label { get; }
+}
diff --git a/src/FluentMermaid/EntityRelationship/Nodes/EntityNode.cs b/src/FluentMermaid/EntityRelationship/Nodes/EntityNode.cs
new file mode 100644
index 0000000..1ccc040
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Nodes/EntityNode.cs
@@ -0,0 +1,31 @@
+using System.Text;
+using FluentMermaid.EntityRelationship.Interfaces;
+
+namespace FluentMermaid.EntityRelationship.Nodes;
+
+internal class EntityNode : IEntity
+{
+    private readonly List<IField> _fields = new();
+
+    public EntityNode(string name)
+    {
+        Name = name ?? throw new ArgumentNullException(nameof(name));
+    }
+
+    public string Name { get; }
+
+    public IField AddField(string type, string name, string? modifier)
+    {
+        var field = new FieldNode(type, name, modifier);
+        _fields.Add(field);
+        return field;
+    }
+
+    public void RenderTo(StringBuilder builder)
+    {
+        builder.AppendLine($"{Name} {{");
+        foreach (IField field in _fields)
+            field.RenderTo(builder);
+        builder.AppendLine("}");
+    }
+}
diff --git a/src/FluentMermaid/EntityRelationship/Nodes/EntityRelationshipDiagramRoot.cs b/src/FluentMermaid/EntityRelationship/Nodes/EntityRelationshipDiagramRoot.cs
new file mode 100644
index 0000000..0d5027d
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Nodes/EntityRelationshipDiagramRoot.cs
@@ -0,0 +1,36 @@
+using System.Text;
+using FluentMermaid.EntityRelationship.Enums;
+using FluentMermaid.EntityRelationship.Interfaces;
+
+namespace FluentMermaid.EntityRelationship.Nodes;
+
+internal class EntityRelationshipDiagramRoot : IEntityRelationshipDiagram
+{
+    private readonly List<IEntity> _entities = new();
+    private readonly List<IRelation> _relations = new();
+
+    public IEntity AddEntity(string name)
+    {
+        var entity = new EntityNode(name);
+        _entities.Add(entity);
+        return entity;
+    }
+
+    public IRelation Relation(IEntity from, IEntity to, Cardinality fromCardinality, Cardinality toCardinality, RelationType relationType, string? label)
+    {
+        var relation = new RelationNode(from, to, fromCardinality, toCardinality, relationType, label);
+        _relations.Add(relation);
+        return relation;
+    }
+
+    public string Render()
+    {
+        var builder = new StringBuilder();
+        builder.AppendLine("erDiagram");
+        foreach (IEntity entity in _entities)
+            entity.RenderTo(builder);
+        foreach (IRelation relation in _relations)
+            relation.RenderTo(builder);
+        return builder.ToString();
+    }
+}
diff --git a/src/FluentMermaid/EntityRelationship/Nodes/FieldNode.cs b/src/FluentMermaid/EntityRelationship/Nodes/FieldNode.cs
new file mode 100644
index 0000000..7179160
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Nodes/FieldNode.cs
@@ -0,0 +1,19 @@
+using System.Text;
+using FluentMermaid.EntityRelationship.Interfaces;
+
+namespace FluentMermaid.EntityRelationship.Nodes;
+
+internal record FieldNode(string Type, string Name, string? Modifier) : IField
+{
+    public void RenderTo(StringBuilder builder)
+    {
+        builder
+            .Append(' ') // indentation
+            .Append(Type)
+            .Append(' ')
+            .Append(Name);
+        if (!string.IsNullOrWhiteSpace(Modifier))
+            builder.Append(' ').Append(Modifier);
+        builder.AppendLine();
+    }
+}
diff --git a/src/FluentMermaid/EntityRelationship/Nodes/RelationNode.cs b/src/FluentMermaid/EntityRelationship/Nodes/RelationNode.cs
new file mode 100644
index 0000000..913cee1
--- /dev/null
+++ b/src/FluentMermaid/EntityRelationship/Nodes/RelationNode.cs
@@ -0,0 +1,41 @@
+using System.Text;
+using FluentMermaid.EntityRelationship.Enums;
+using FluentMermaid.EntityRelationship.Extensions;
+using FluentMermaid.EntityRelationship.Interfaces;
+
+namespace FluentMermaid.EntityRelationship.Nodes;
+
+internal class RelationNode : IRelation
+{
+    public RelationNode(IEntity from, IEntity to, Cardinality fromCardinality, Cardinality toCardinality, RelationType relationType, string? label)
+    {
+        From = from;
+        To = to;
+        FromCardinality = fromCardinality;
+        ToCardinality = toCardinality;
+        RelationType = relationType;
+        Label = label;
+    }
+
+    public IEntity From { get; }
+    public IEntity To { get; }
+    public Cardinality FromCardinality { get; }
+    public Cardinality ToCardinality { get; }
+    public RelationType RelationType { get; }
+    public string? Label { get; }
+
+    public void RenderTo(StringBuilder builder)
+    {
+        builder
+            .Append(From.Name)
+            .Append(' ')
+            .Append(FromCardinality.Render(true))
+            .Append(RelationType.Render())
+            .Append(ToCardinality.Render(false))
+            .Append(' ')
+            .Append(To.Name);
+        if (!string.IsNullOrWhiteSpace(Label))
+            builder.Append(" : ").Append(Label);
+        builder.AppendLine();
+    }
+}