diff --git a/.github/copilot-tasks.md b/.github/copilot-tasks.md
index 1bce20e..aa222bf 100644
--- a/.github/copilot-tasks.md
+++ b/.github/copilot-tasks.md
@@ -59,12 +59,12 @@ Projekt vollstaendig umsetzen: aus PS_PW.xlsx und Stellenbeschreibungen.xlsx mon
 	- [x] Monatsmarker parsebar
 	- [x] Gewuenschter Monat exakt einmal vorhanden
 	- [x] Stunden-/Vertragswerte gueltig
-- [ ] T10: Business-Validierung implementieren
-	- [ ] Mindestens ein aktiver Fall vorhanden
-	- [ ] Jeder aktive Fall hat eindeutigen Match oder klaren Fehler
-	- [ ] Template enthaelt genau ein Quellblatt
-	- [ ] Dateinamenkollisionen vermeiden
-	- [ ] Pflicht-Zielzellen vollstaendig konfiguriert
+- [x] T10: Business-Validierung implementieren
+	- [x] Mindestens ein aktiver Fall vorhanden
+	- [x] Jeder aktive Fall hat eindeutigen Match oder klaren Fehler
+	- [x] Template enthaelt genau ein Quellblatt
+	- [x] Dateinamenkollisionen vermeiden
+	- [x] Pflicht-Zielzellen vollstaendig konfiguriert
 
 ## Phase 6 - Ausgabeerzeugung
 - [ ] T11: Template-Verarbeitung bauen
@@ -111,4 +111,4 @@ Projekt vollstaendig umsetzen: aus PS_PW.xlsx und Stellenbeschreibungen.xlsx mon
 8. T15 -> T16 -> T17
 
 ## Aktueller Startpunkt
-- [ ] Naechste aktive Aufgabe: T10 (Business-Validierung implementieren)
+- [ ] Naechste aktive Aufgabe: T11 (Template-Verarbeitung bauen)
diff --git a/src/taetigkeitsnachweis/validator.py b/src/taetigkeitsnachweis/validator.py
index b2e47d6..8918c91 100644
--- a/src/taetigkeitsnachweis/validator.py
+++ b/src/taetigkeitsnachweis/validator.py
@@ -9,7 +9,7 @@ from typing import Any
 from openpyxl import load_workbook
 from openpyxl.utils.cell import column_index_from_string
 
-from .models import ValidationIssue
+from .models import AllocationRecord, MatchedAssignment, ValidationIssue
 
 
 def _to_text(value: Any) -> str:
@@ -232,3 +232,141 @@ def validate_sources(
     allocation_wb.close()
     master_wb.close()
     return issues
+
+
+def _split_name_for_filename(name: str) -> tuple[str, str]:
+    cleaned = _to_text(name)
+    if "," in cleaned:
+        last_name, first_name = [part.strip() for part in cleaned.split(",", 1)]
+        return first_name or "Unknown", last_name or "Unknown"
+
+    parts = cleaned.split()
+    if len(parts) >= 2:
+        return parts[0], parts[-1]
+    if len(parts) == 1:
+        return parts[0], "Unknown"
+    return "Unknown", "Unknown"
+
+
+def _build_output_filename(pattern: str, year: int, month: int, employee_name: str) -> str:
+    first_name, last_name = _split_name_for_filename(employee_name)
+    safe_first = first_name.replace(" ", "_")
+    safe_last = last_name.replace(" ", "_")
+    return pattern.format(
+        year=year,
+        month=f"{month:02d}",
+        first_name=safe_first,
+        last_name=safe_last,
+    )
+
+
+def validate_business_rules(
+    config: dict[str, Any],
+    report_month: int,
+    report_year: int,
+    allocation_records: list[AllocationRecord],
+    matched_assignments: list[MatchedAssignment],
+) -> list[ValidationIssue]:
+    issues: list[ValidationIssue] = []
+
+    if len(allocation_records) == 0:
+        issues.append(
+            ValidationIssue(
+                code="NO_ACTIVE_ASSIGNMENT",
+                message="No active allocation record found for the requested month.",
+                severity="error",
+            )
+        )
+
+    matched_keys: set[tuple[str, str, float]] = set()
+    for item in matched_assignments:
+        key = (
+            item.allocation.employee_name,
+            item.allocation.project_key,
+            item.allocation.funded_hours_weekly,
+        )
+        if key in matched_keys:
+            issues.append(
+                ValidationIssue(
+                    code="DUPLICATE_MATCH",
+                    message=(
+                        f"Duplicate matched assignment for '{item.allocation.employee_name}' "
+                        f"and project '{item.allocation.project_key}'."
+                    ),
+                    severity="error",
+                    context=item.allocation.employee_name,
+                )
+            )
+        matched_keys.add(key)
+
+    paths_cfg = config["paths"]
+    files_cfg = config["files"]
+    project_root = Path.cwd()
+    template_path = (
+        project_root
+        / paths_cfg["template_dir"]
+        / files_cfg["template_workbook"]
+    )
+
+    if template_path.exists():
+        template_wb = load_workbook(template_path, data_only=True)
+        if len(template_wb.sheetnames) != 1:
+            issues.append(
+                ValidationIssue(
+                    code="TEMPLATE_SHEET_COUNT",
+                    message=(
+                        f"Template workbook must contain exactly one sheet, found {len(template_wb.sheetnames)}."
+                    ),
+                    severity="error",
+                )
+            )
+        template_wb.close()
+    else:
+        issues.append(
+            ValidationIssue(
+                code="MISSING_TEMPLATE",
+                message=f"Template workbook not found: {template_path}",
+                severity="error",
+            )
+        )
+
+    template_targets = config.get("template_targets", {})
+    missing_targets = [name for name, cell in template_targets.items() if not _to_text(cell)]
+    if missing_targets:
+        issues.append(
+            ValidationIssue(
+                code="MISSING_TEMPLATE_TARGETS",
+                message=(
+                    "Mandatory template target cells not configured: "
+                    + ", ".join(missing_targets)
+                ),
+                severity="error",
+            )
+        )
+
+    filename_pattern = config["output"]["filename_pattern"]
+    filename_to_employee: dict[str, str] = {}
+    for assignment in matched_assignments:
+        filename = _build_output_filename(
+            filename_pattern,
+            report_year,
+            report_month,
+            assignment.allocation.employee_name,
+        )
+        previous_owner = filename_to_employee.get(filename)
+        if previous_owner is None:
+            filename_to_employee[filename] = assignment.allocation.employee_name
+            continue
+        if previous_owner != assignment.allocation.employee_name:
+            issues.append(
+                ValidationIssue(
+                    code="FILENAME_COLLISION",
+                    message=(
+                        f"Output filename collision between '{previous_owner}' and "
+                        f"'{assignment.allocation.employee_name}' -> {filename}"
+                    ),
+                    severity="error",
+                )
+            )
+
+    return issues