diff --git a/Sources/BetterFit/Models/DailyHealthSummaryExport.swift b/Sources/BetterFit/Models/DailyHealthSummaryExport.swift
new file mode 100644
index 0000000..b23c07c
--- /dev/null
+++ b/Sources/BetterFit/Models/DailyHealthSummaryExport.swift
@@ -0,0 +1,64 @@
+import Foundation
+
+// MARK: - DailyHealthSummary Export
+
+extension Array where Element == DailyHealthSummary {
+
+    /// CSV header shared by every exported file.
+    public static var dailyHealthSummaryCSVHeader: String {
+        "date,step_count,active_energy_kcal,basal_energy_kcal,distance_metres,"
+            + "average_heart_rate,resting_heart_rate,body_mass_kg,sleep_duration_seconds"
+    }
+
+    /// Encode the summaries as CSV, one row per day in array order.
+    ///
+    /// Dates use ISO8601. Missing values are empty fields so a nil never
+    /// reads as a measured zero. Rows end with a newline, including the last.
+    public func csvString() -> String {
+        var lines: [String] = [Self.dailyHealthSummaryCSVHeader]
+        let formatter = ISO8601DateFormatter()
+        for summary in self {
+            lines.append(
+                [
+                    formatter.string(from: summary.date),
+                    Self.csvNumber(summary.stepCount),
+                    Self.csvNumber(summary.activeEnergyKilocalories),
+                    Self.csvNumber(summary.basalEnergyKilocalories),
+                    Self.csvNumber(summary.distanceMetres),
+                    Self.csvNumber(summary.averageHeartRate),
+                    Self.csvNumber(summary.restingHeartRate),
+                    Self.csvNumber(summary.bodyMassKilograms),
+                    Self.csvNumber(summary.sleepDuration)
+                ].joined(separator: ",")
+            )
+        }
+        return lines.joined(separator: "\n") + "\n"
+    }
+
+    /// Encode the summaries as pretty printed JSON with sorted keys and
+    /// ISO8601 dates. Decoding the result gives back the same summaries.
+    public func jsonData() throws -> Data {
+        let encoder = JSONEncoder()
+        encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
+        encoder.dateEncodingStrategy = .iso8601
+        return try encoder.encode(self)
+    }
+
+    /// Write the CSV form to `url`, creating or replacing the file.
+    public func writeCSV(to url: URL) throws {
+        try csvString().write(to: url, atomically: true, encoding: .utf8)
+    }
+
+    /// Write the JSON form to `url`, creating or replacing the file.
+    public func writeJSON(to url: URL) throws {
+        try jsonData().write(to: url, options: .atomic)
+    }
+
+    private static func csvNumber(_ value: Double?) -> String {
+        guard let value else { return "" }
+        if value == value.rounded(), abs(value) < 1e15 {
+            return String(Int(value))
+        }
+        return String(value)
+    }
+}
diff --git a/Tests/BetterFitTests/DailyHealthSummaryExportTests.swift b/Tests/BetterFitTests/DailyHealthSummaryExportTests.swift
new file mode 100644
index 0000000..ac5b84e
--- /dev/null
+++ b/Tests/BetterFitTests/DailyHealthSummaryExportTests.swift
@@ -0,0 +1,135 @@
+import XCTest
+
+@testable import BetterFit
+
+final class DailyHealthSummaryExportTests: XCTestCase {
+
+    // MARK: - Fixtures
+
+    private func date(_ string: String) -> Date {
+        let formatter = ISO8601DateFormatter()
+        return formatter.date(from: string)!
+    }
+
+    private var sample: [DailyHealthSummary] {
+        [
+            DailyHealthSummary(
+                date: date("2026-04-19T00:00:00Z"),
+                stepCount: 8000,
+                activeEnergyKilocalories: 450.5,
+                basalEnergyKilocalories: 1600,
+                distanceMetres: 5200,
+                averageHeartRate: 82.5,
+                restingHeartRate: 58,
+                bodyMassKilograms: 72.5,
+                sleepDuration: 25200
+            ),
+            DailyHealthSummary(date: date("2026-04-20T00:00:00Z"), stepCount: 1200)
+        ]
+    }
+
+    // MARK: - CSV
+
+    func testCSVHeaderAndRows() {
+        let csv = sample.csvString()
+        let lines = csv.split(separator: "\n", omittingEmptySubsequences: false)
+        XCTAssertEqual(
+            String(lines[0]),
+            "date,step_count,active_energy_kcal,basal_energy_kcal,distance_metres,"
+                + "average_heart_rate,resting_heart_rate,body_mass_kg,sleep_duration_seconds"
+        )
+        XCTAssertEqual(
+            String(lines[1]),
+            "2026-04-19T00:00:00Z,8000,450.5,1600,5200,82.5,58,72.5,25200"
+        )
+        XCTAssertTrue(csv.hasSuffix("\n"))
+    }
+
+    func testCSVEmptyFieldsForMissingValues() {
+        let csv = sample.csvString()
+        let lines = csv.split(separator: "\n")
+        XCTAssertEqual(String(lines[2]), "2026-04-20T00:00:00Z,1200,,,,,,,")
+    }
+
+    func testCSVEmptyArrayIsHeaderOnly() {
+        XCTAssertEqual(
+            [DailyHealthSummary]().csvString(),
+            [DailyHealthSummary].dailyHealthSummaryCSVHeader + "\n"
+        )
+    }
+
+    func testCSVPreservesArrayOrder() {
+        let csv = Array(sample.reversed()).csvString()
+        let lines = csv.split(separator: "\n")
+        XCTAssertTrue(String(lines[1]).hasPrefix("2026-04-20"))
+        XCTAssertTrue(String(lines[2]).hasPrefix("2026-04-19"))
+    }
+
+    // MARK: - JSON
+
+    func testJSONRoundTrip() throws {
+        let data = try sample.jsonData()
+        let decoder = JSONDecoder()
+        decoder.dateDecodingStrategy = .iso8601
+        let decoded = try decoder.decode([DailyHealthSummary].self, from: data)
+        XCTAssertEqual(decoded, sample)
+    }
+
+    func testJSONUsesISO8601DatesAndSortedKeys() throws {
+        let json = String(data: try sample.jsonData(), encoding: .utf8)!
+        XCTAssertTrue(json.contains("\"2026-04-19T00:00:00Z\""))
+        // Sorted keys put activeEnergyKilocalories before date in each object.
+        let firstActive = json.range(of: "activeEnergyKilocalories")!.lowerBound
+        let firstDate = json.range(of: "\"date\"")!.lowerBound
+        XCTAssertTrue(firstActive < firstDate)
+    }
+
+    func testJSONEmptyArray() throws {
+        let data = try [DailyHealthSummary]().jsonData()
+        let decoder = JSONDecoder()
+        decoder.dateDecodingStrategy = .iso8601
+        XCTAssertEqual(try decoder.decode([DailyHealthSummary].self, from: data), [])
+    }
+
+    // MARK: - File Writing
+
+    func testWriteCSVAndJSONToDisk() throws {
+        let dir = FileManager.default.temporaryDirectory
+            .appendingPathComponent(UUID().uuidString)
+        try FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: dir) }
+
+        let csvURL = dir.appendingPathComponent("daily.csv")
+        try sample.writeCSV(to: csvURL)
+        XCTAssertEqual(try String(contentsOf: csvURL, encoding: .utf8), sample.csvString())
+
+        let jsonURL = dir.appendingPathComponent("daily.json")
+        try sample.writeJSON(to: jsonURL)
+        XCTAssertEqual(try Data(contentsOf: jsonURL), try sample.jsonData())
+    }
+
+    // MARK: - End to End
+
+    func testExportFromParsedExport() throws {
+        let xml = """
+            <?xml version="1.0" encoding="UTF-8"?>
+            <HealthData locale="en_AU">
+              <Record type="HKQuantityTypeIdentifierStepCount" unit="count" \
+            value="3000" startDate="2026-04-19 08:00:00 +0000" endDate="2026-04-19 09:00:00 +0000"/>
+            </HealthData>
+            """
+        var calendar = Calendar(identifier: .gregorian)
+        calendar.timeZone = TimeZone(secondsFromGMT: 0)!
+        let summaries = try HealthExportParser.parseExport(xml: xml)
+            .dailySummaries(calendar: calendar)
+        XCTAssertEqual(summaries.count, 1)
+        XCTAssertTrue(summaries.csvString().contains("2026-04-19T00:00:00Z,3000"))
+        let decoder = JSONDecoder()
+        decoder.dateDecodingStrategy = .iso8601
+        let decoded = try decoder.decode(
+            [DailyHealthSummary].self,
+            from: summaries.jsonData()
+        )
+        XCTAssertEqual(decoded, summaries)
+    }
+}