diff --git a/Sources/KiteCore/Builtins.swift b/Sources/KiteCore/Builtins.swift
new file mode 100644
index 0000000..0e31083
--- /dev/null
+++ b/Sources/KiteCore/Builtins.swift
@@ -0,0 +1,136 @@
+import Foundation
+
+let preludeSource = """
+(define-syntax when
+  (syntax-rules ()
+    ((_ test body ...) (if test (begin body ...) #f))))
+
+(define-syntax unless
+  (syntax-rules ()
+    ((_ test body ...) (if test #f (begin body ...)))))
+"""
+
+func installBuiltins(into env: Environment, interp: Interpreter) {
+    func def(_ name: String, _ special: SpecialKind = .none, _ fn: @escaping ([Value]) throws -> Value) {
+        env.define(name, .builtin(Builtin(name: name, special: special, fn: fn)))
+    }
+
+    def("+") { args in
+        var acc = Num.int(0)
+        for a in args { acc = numAdd(acc, try asNum(a, "+")) }
+        return numToValue(acc)
+    }
+    def("*") { args in
+        var acc = Num.int(1)
+        for a in args { acc = numMul(acc, try asNum(a, "*")) }
+        return numToValue(acc)
+    }
+    def("-") { args in
+        guard let first = args.first else { throw schemeError("-: needs at least 1 argument") }
+        if args.count == 1 {
+            return numToValue(numSub(.int(0), try asNum(first, "-")))
+        }
+        var acc = try asNum(first, "-")
+        for a in args[1...] { acc = numSub(acc, try asNum(a, "-")) }
+        return numToValue(acc)
+    }
+    def("/") { args in
+        guard let first = args.first else { throw schemeError("/: needs at least 1 argument") }
+        if args.count == 1 {
+            return numToValue(try numDiv(.int(1), try asNum(first, "/")))
+        }
+        var acc = try asNum(first, "/")
+        for a in args[1...] { acc = try numDiv(acc, try asNum(a, "/")) }
+        return numToValue(acc)
+    }
+
+    func chainCompare(_ name: String, _ pred: @escaping (Int) -> Bool) {
+        def(name) { args in
+            for i in 1..<max(args.count, 1) where i < args.count {
+                let c = numCompare(try asNum(args[i - 1], name), try asNum(args[i], name))
+                if !pred(c) { return .bool(false) }
+            }
+            return .bool(true)
+        }
+    }
+    chainCompare("=") { $0 == 0 }
+    chainCompare("<") { $0 < 0 }
+    chainCompare(">") { $0 > 0 }
+    chainCompare("<=") { $0 <= 0 }
+    chainCompare(">=") { $0 >= 0 }
+
+    func intArg(_ v: Value, _ who: String) throws -> Int {
+        guard case .integer(let n) = v else { throw schemeError("\(who): expected integer", [v]) }
+        return n
+    }
+    def("modulo") { args in
+        let a = try intArg(args[0], "modulo")
+        let b = try intArg(args[1], "modulo")
+        if b == 0 { throw schemeError("modulo: division by zero") }
+        let r = a % b
+        return .integer((r != 0 && (r < 0) != (b < 0)) ? r + b : r)
+    }
+    def("remainder") { args in
+        let a = try intArg(args[0], "remainder")
+        let b = try intArg(args[1], "remainder")
+        if b == 0 { throw schemeError("remainder: division by zero") }
+        return .integer(a % b)
+    }
+    def("quotient") { args in
+        let a = try intArg(args[0], "quotient")
+        let b = try intArg(args[1], "quotient")
+        if b == 0 { throw schemeError("quotient: division by zero") }
+        return .integer(a / b)
+    }
+    def("abs") { args in
+        switch args[0] {
+        case .integer(let n): return .integer(abs(n))
+        case .real(let d): return .real(abs(d))
+        case .rational(let n, let d): return makeRational(abs(n), d)
+        default: throw schemeError("abs: expected number", [args[0]])
+        }
+    }
+    def("min") { args in
+        var best = try asNum(args[0], "min")
+        for a in args[1...] {
+            let n = try asNum(a, "min")
+            if numCompare(n, best) < 0 { best = n }
+        }
+        return numToValue(best)
+    }
+    def("max") { args in
+        var best = try asNum(args[0], "max")
+        for a in args[1...] {
+            let n = try asNum(a, "max")
+            if numCompare(n, best) > 0 { best = n }
+        }
+        return numToValue(best)
+    }
+    def("gcd") { args in
+        var acc = 0
+        for a in args { acc = gcdInt(acc, try intArg(a, "gcd")) }
+        return .integer(acc)
+    }
+    def("expt") { args in
+        if case .integer(let base) = args[0], case .integer(let exp) = args[1], exp >= 0 {
+            var result = 1
+            var overflow = false
+            for _ in 0..<exp {
+                let (r, o) = result.multipliedReportingOverflow(by: base)
+                if o { overflow = true; break }
+                result = r
+            }
+            if !overflow { return .integer(result) }
+        }
+        let base = numToDouble(try asNum(args[0], "expt"))
+        let exp = numToDouble(try asNum(args[1], "expt"))
+        return .real(pow(base, exp))
+    }
+    def("zero?") { args in .bool(numCompare(try asNum(args[0], "zero?"), .int(0)) == 0) }
+    def("positive?") { args in .bool(numCompare(try asNum(args[0], "positive?"), .int(0)) > 0) }
+    def("negative?") { args in .bool(numCompare(try asNum(args[0], "negative?"), .int(0)) < 0) }
+    def("even?") { args in .bool(try intArg(args[0], "even?") % 2 == 0) }
+    def("odd?") { args in .bool(try intArg(args[0], "odd?") % 2 != 0) }
+    def("1+") { args in numToValue(numAdd(try asNum(args[0], "1+"), .int(1))) }
+    def("1-") { args in numToValue(numSub(try asNum(args[0], "1-"), .int(1))) }
+}