diff --git a/auth.go b/auth.go
index cbe9a13..6face6a 100644
--- a/auth.go
+++ b/auth.go
@@ -1,12 +1,52 @@
 package writeas
 
 import (
+	"fmt"
 	"net/http"
 )
 
+// LogIn authenticates a user with Write.as.
+// See https://writeas.github.io/docs/#authenticate-a-user
+func (c *Client) LogIn(username, pass string) (*AuthUser, error) {
+	u := &AuthUser{}
+	up := struct {
+		Alias string `json:"alias"`
+		Pass  string `json:"pass"`
+	}{
+		Alias: username,
+		Pass:  pass,
+	}
+
+	env, err := c.post("/auth/login", up, u)
+	if err != nil {
+		return nil, err
+	}
+
+	var ok bool
+	if u, ok = env.Data.(*AuthUser); !ok {
+		return nil, fmt.Errorf("Wrong data returned from API.")
+	}
+
+	status := env.Code
+	if status == http.StatusOK {
+		return u, nil
+	} else if status == http.StatusBadRequest {
+		return nil, fmt.Errorf("Bad request: %s", env.ErrorMessage)
+	} else if status == http.StatusUnauthorized {
+		return nil, fmt.Errorf("Incorrect password.")
+	} else if status == http.StatusNotFound {
+		return nil, fmt.Errorf("User does not exist.")
+	} else if status == http.StatusTooManyRequests {
+		return nil, fmt.Errorf("Stop repeatedly trying to log in.")
+	} else {
+		return nil, fmt.Errorf("Problem authenticating: %s. %v\n", status, err)
+	}
+	return u, nil
+}
+
 func (c *Client) isNotLoggedIn(code int) bool {
 	if c.token == "" {
 		return false
 	}
 	return code == http.StatusUnauthorized
 }
diff --git a/user.go b/user.go
new file mode 100644
index 0000000..bc5c4a1
--- /dev/null
+++ b/user.go
@@ -0,0 +1,22 @@
+package writeas
+
+import (
+	"time"
+)
+
+type (
+	// AuthUser represents a just-authenticated user. It contains information
+	// that'll only be returned once (now) per user session.
+	AuthUser struct {
+		AccessToken string `json:"access_token,omitempty"`
+		Password    string `json:"password,omitempty"`
+		User        *User  `json:"user"`
+	}
+
+	// User represents a registered Write.as user.
+	User struct {
+		Username string    `json:"username"`
+		Email    string    `json:"email"`
+		Created  time.Time `json:"created"`
+	}
+)