diff --git a/msp/errors.go b/msp/errors.go
new file mode 100644
index 0000000..ff500f7
--- /dev/null
+++ b/msp/errors.go
@@ -0,0 +1,12 @@
+package msp
+
+import (
+	"errors"
+)
+
+var (
+	// ErrEndAfterStart occurs when a period given has an end time after its start time
+	ErrEndAfterStart = errors.New("error: start time is after end time")
+	// ErrNoValidPeriods occurs when an empty set of periods is passed or when ll periods are invalid
+	ErrNoValidPeriods = errors.New("error: no valid periods available")
+)
diff --git a/msp/msp.go b/msp/msp.go
index 409bd44..12dc73f 100644
--- a/msp/msp.go
+++ b/msp/msp.go
@@ -1,10 +1,69 @@
 package msp
 
-var ()
+import (
+	"sort"
+	"time"
+)
 
-func init() {
+func MostSpecificPeriod(ts time.Time, periods ...Period) (id string, err error) {
+	// Filter to get only valid periods here
+	periods = ValidTimePeriods(ts, periods...)
+	if len(periods) == 0 {
+		return "", ErrNoValidPeriods
+	}
+	// find the shortest duration
+	d, err := GetDuration(periods[0].StartTime, periods[0].EndTime)
+	for _, x := range periods {
+		p, err := GetDuration(x.StartTime, x.EndTime)
+		if err == nil && p < d {
+			d = p
+		}
+	}
+	// find all periods with this shortest duration
+	var matchingDurations []Period
+	for _, x := range periods {
+		p, err := GetDuration(x.StartTime, x.EndTime)
+		if err == nil && p == d {
+			matchingDurations = append(matchingDurations, x)
+		}
+	}
+	// Find the newest time a period starts
+	newest := matchingDurations[0].StartTime
+	for _, x := range matchingDurations {
+		if x.StartTime.After(newest) {
+			newest = x.StartTime
+		}
+	}
+	// Determine whichever of these periods have the same start time in addtion to duration
+	var matchingDurationsAndStartTimes []Period
+	for _, x := range matchingDurations {
+		if x.StartTime == newest {
+			matchingDurationsAndStartTimes = append(matchingDurationsAndStartTimes, x)
+		}
+	}
+	// Finally, return the period with the 'last' name lexicographically
+	var identifiers []string
+	for _, x := range matchingDurationsAndStartTimes {
+		identifiers = append(identifiers, x.Identifier)
+	}
+	sort.Strings(identifiers)
+	return identifiers[len(identifiers)-1], nil
 }
 
-func MostSpecificPeriod(periods ...Period) (id string, err error) {
-	return
+func GetDuration(start time.Time, end time.Time) (dur time.Duration, err error) {
+	if start.After(end) {
+		err = ErrEndAfterStart
+	}
+	dur = end.Sub(start)
+	return dur, err
+}
+
+func ValidTimePeriods(ts time.Time, periods ...Period) []Period {
+	var valid []Period
+	for _, p := range periods {
+		if p.StartTime.Before(ts) && p.EndTime.After(ts) {
+			valid = append(valid, p)
+		}
+	}
+	return valid
 }
diff --git a/msp/msp_test.go b/msp/msp_test.go
index cccf3a6..3eff177 100644
--- a/msp/msp_test.go
+++ b/msp/msp_test.go
@@ -1,9 +1,20 @@
 package msp
 
 import (
+	"fmt"
 	"testing"
 )
 
 //(periods ...Period) (id string, err error) {
 func TestMostSpecificPeriod(t *testing.T) {
+	testCases := []struct {
+		testID  string
+		result  string
+		Periods []Period
+	}{{testID: "test"}}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%s", tc.testID), func(t *testing.T) {
+
+		})
+	}
 }