diff --git a/Makefile.am b/Makefile.am
index 914c558..09a52d5 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -63,6 +63,7 @@
 	sensors/host.cpp \
 	sensors/builder.cpp \
 	sensors/builderconfig.cpp \
+	sensors/buildjson.cpp \
 	sensors/manager.cpp \
 	pid/ec/pid.cpp \
 	pid/ec/stepwise.cpp \
diff --git a/sensors/buildjson.cpp b/sensors/buildjson.cpp
new file mode 100644
index 0000000..7c4c7d6
--- /dev/null
+++ b/sensors/buildjson.cpp
@@ -0,0 +1,88 @@
+/**
+ * Copyright 2019 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "sensors/buildjson.hpp"
+
+#include "conf.hpp"
+#include "sensors/sensor.hpp"
+
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+void from_json(const json& j, SensorConfig& s)
+{
+    j.at("type").get_to(s.type);
+    j.at("readpath").get_to(s.readpath);
+
+    /* The writepath field is optional in a configuration */
+    auto writepath = j.find("writepath");
+    if (writepath == j.end())
+    {
+        s.writepath = "";
+    }
+    else
+    {
+        j.at("writepath").get_to(s.writepath);
+    }
+
+    /* The min field is optional in a configuration. */
+    auto min = j.find("min");
+    if (min == j.end())
+    {
+        s.min = 0;
+    }
+    else
+    {
+        j.at("min").get_to(s.min);
+    }
+
+    /* The max field is optional in a configuration. */
+    auto max = j.find("max");
+    if (max == j.end())
+    {
+        s.max = 0;
+    }
+    else
+    {
+        j.at("max").get_to(s.max);
+    }
+
+    /* The timeout field is optional in a configuration. */
+    auto timeout = j.find("timeout");
+    if (timeout == j.end())
+    {
+        s.timeout = Sensor::getDefaultTimeout(s.type);
+    }
+    else
+    {
+        j.at("timeout").get_to(s.timeout);
+    }
+}
+
+std::map<std::string, struct SensorConfig>
+    buildSensorsFromJson(const json& data)
+{
+    std::map<std::string, struct SensorConfig> config;
+    auto sensors = data["sensors"];
+
+    for (const auto& sensor : sensors)
+    {
+        config[sensor["name"]] = sensor.get<struct SensorConfig>();
+    }
+
+    return config;
+}
diff --git a/sensors/buildjson.hpp b/sensors/buildjson.hpp
new file mode 100644
index 0000000..c5aacc7
--- /dev/null
+++ b/sensors/buildjson.hpp
@@ -0,0 +1,20 @@
+#pragma once
+
+#include "conf.hpp"
+
+#include <map>
+#include <nlohmann/json.hpp>
+#include <string>
+
+using json = nlohmann::json;
+
+/**
+ * Given a json object generated from a configuration file, build the sensor
+ * configuration representation. This expecteds the json configuration to be
+ * valid.
+ *
+ * @param[in] data - the json data
+ * @return a map of sensors.
+ */
+std::map<std::string, struct SensorConfig>
+    buildSensorsFromJson(const json& data);
diff --git a/test/Makefile.am b/test/Makefile.am
index c46284f..fb1d58d 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -17,7 +17,8 @@
  sensor_host_unittest util_unittest pid_zone_unittest \
  pid_thermalcontroller_unittest pid_fancontroller_unittest \
  pid_stepwisecontroller_unittest \
- dbus_passive_unittest dbus_active_unittest
+ dbus_passive_unittest dbus_active_unittest \
+ sensors_json_unittest
 TESTS = $(check_PROGRAMS)
 
 # Until libconfig is mocked out or replaced, include it.
@@ -58,3 +59,6 @@
 
 dbus_active_unittest_SOURCES = dbus_active_unittest.cpp
 dbus_active_unittest_LDADD = $(top_builddir)/dbus/dbusactiveread.o
+
+sensors_json_unittest_SOURCES = sensors_json_unittest.cpp
+sensors_json_unittest_LDADD = $(top_builddir)/sensors/buildjson.o
diff --git a/test/sensors_json_unittest.cpp b/test/sensors_json_unittest.cpp
new file mode 100644
index 0000000..99ab3db
--- /dev/null
+++ b/test/sensors_json_unittest.cpp
@@ -0,0 +1,100 @@
+#include "sensors/buildjson.hpp"
+#include "sensors/sensor.hpp"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+TEST(SensorsFromJson, emptyJsonNoSensors)
+{
+    // If the json has no sensors, the map is empty.
+
+    auto j2 = R"(
+      {
+        "sensors": []
+      }
+    )"_json;
+
+    auto output = buildSensorsFromJson(j2);
+    EXPECT_TRUE(output.empty());
+}
+
+TEST(SensorsFromJson, oneFanSensor)
+{
+    // If the json has one sensor, it's in the map.
+
+    auto j2 = R"(
+      {
+        "sensors": [{
+            "name": "fan1",
+            "type": "fan",
+            "readpath": "/xyz/openbmc_project/sensors/fan_tach/fan1",
+            "writepath": "/sys/devices/platform/ahb/ahb:apb/1e786000.pwm-tacho-controller/hwmon/**/pwm1",
+            "min": 0,
+            "max": 255
+        }]
+      }
+    )"_json;
+
+    auto output = buildSensorsFromJson(j2);
+    EXPECT_EQ(1, output.size());
+    EXPECT_EQ(output["fan1"].type, "fan");
+    EXPECT_EQ(output["fan1"].readpath,
+              "/xyz/openbmc_project/sensors/fan_tach/fan1");
+    EXPECT_EQ(output["fan1"].writepath,
+              "/sys/devices/platform/ahb/ahb:apb/1e786000.pwm-tacho-controller/"
+              "hwmon/**/pwm1");
+    EXPECT_EQ(output["fan1"].min, 0);
+    EXPECT_EQ(output["fan1"].max, 255);
+    EXPECT_EQ(output["fan1"].timeout,
+              Sensor::getDefaultTimeout(output["fan1"].type));
+}
+
+TEST(SensorsFromJson, validateOptionalFields)
+{
+    // The writepath, min, max, timeout fields are optional.
+
+    auto j2 = R"(
+      {
+        "sensors": [{
+            "name": "fan1",
+            "type": "fan",
+            "readpath": "/xyz/openbmc_project/sensors/fan_tach/fan1"
+        }]
+      }
+    )"_json;
+
+    auto output = buildSensorsFromJson(j2);
+    EXPECT_EQ(1, output.size());
+    EXPECT_EQ(output["fan1"].type, "fan");
+    EXPECT_EQ(output["fan1"].readpath,
+              "/xyz/openbmc_project/sensors/fan_tach/fan1");
+    EXPECT_EQ(output["fan1"].writepath, "");
+    EXPECT_EQ(output["fan1"].min, 0);
+    EXPECT_EQ(output["fan1"].max, 0);
+    EXPECT_EQ(output["fan1"].timeout,
+              Sensor::getDefaultTimeout(output["fan1"].type));
+}
+
+TEST(SensorsFromJson, twoSensors)
+{
+    // Same as one sensor, but two.
+    // If a configuration has two sensors with the same name the information
+    // last is the information used.
+
+    auto j2 = R"(
+      {
+        "sensors": [{
+            "name": "fan1",
+            "type": "fan",
+            "readpath": "/xyz/openbmc_project/sensors/fan_tach/fan1"
+        }, {
+            "name": "fan2",
+            "type": "fan",
+            "readpath": "/xyz/openbmc_project/sensors/fan_tach/fan1"
+        }]
+      }
+    )"_json;
+
+    auto output = buildSensorsFromJson(j2);
+    EXPECT_EQ(2, output.size());
+}