diff --git a/src/expression.c b/src/expression.c
index 4f1e87d..354b805 100644
--- a/src/expression.c
+++ b/src/expression.c
@@ -1,11 +1,17 @@
 #include "expression.h"
+#include "fieldset.H"
 
 #include "../lib/xalloc.h"
 
 /* Static helper functions */
 
 static node_t* alloc_node();
-static int eval_single_node(node_t* node);
+static int eval_gt_node(node_t *node, fieldset_t *fields);
+static int eval_lt_node(node_t *node, fieldset_t *fields);
+static int eval_eq_node(node_t *node, fieldset_t *fields);
+static int eval_lt_eq_node(node_t *node, fieldset_t *fields);
+static int eval_gt_eq_node(node_t *node, fieldset_t *fields);
+
 
 static node_t* alloc_node() 
 {
@@ -14,6 +20,54 @@ static node_t* alloc_node()
 	return node;
 }
 
+static int eval_gt_node(node_t *node, fieldset_t *fields)
+{
+	int index = node->left_child->value.field.index;
+	uint64_t expected = node->right_child->value.int_literal;
+	uint64_t actual = fs_get_uint64_by_index(fields, index);
+	return (actual > expected);
+}
+
+static int eval_lt_node(node_t *node, fieldset_t *fields)
+{
+	int index = node->left_child->value.field.index;
+	uint64_t expected = node->right_child->value.int_literal;
+	uint64_t actual = fs_get_uint64_by_index(fields, index);
+	return (actual < expected);
+}
+
+static int eval_eq_node(node_t *node, fieldset_t *fields)
+{
+	node_t *literal = node->right_child;
+	int index = node->left_child->value.field.index;
+	char *expected, *actual;
+	switch (literal->type) {
+		case STRING:
+			expected = literal->value.string_literal;
+			actual = fs_get_string_by_index(fields, index);
+			return (strcmp(expected, actual) == 0);
+			break;
+		case INT:
+			return (fs_get_uint64_by_index(fields, index) == literal->value.int_literal);
+			break;
+		default:
+			printf("wat\n");
+			break;
+	}
+	return 0;
+}
+
+static int eval_lt_eq_node(node_t *node, fieldset_t *fields)
+{
+	return !(eval_gt_node(node, fields));
+}
+
+static int eval_gt_eq_node(node_t *node, fieldset_t *fields)
+{
+	return !(eval_lt_node(node, fields));
+}
+
+
 /* Exposed functions */
 
 node_t* make_op_node(enum operation op) 
@@ -48,10 +102,37 @@ node_t* make_int_node(int literal)
 	return node;
 }
 
-int evaluate_expression(node_t *root) {
-	if (!root) return 0;
-	int result = 1;
-	return result;
+int evaluate_expression(node_t *root, fieldset_t *fields) {
+	if (!root) return 1;
+	switch (root->type) { /* XXX Not sure if runs */
+	case FIELD:
+	case STRING:
+	case INT:
+		return 1;
+	case OP:
+		break;
+	}
+	switch (root->value.op) {
+	case GT:
+		return eval_gt_node(root, fields);
+	case LT:
+		return eval_lt_node(root, fields);
+	case EQ:
+		return eval_eq_node(root, fields);
+	case NEQ:
+		return (!eval_eq_node(root, fields));
+	case LT_EQ:
+		return eval_lt_eq_node(root, fields);
+	case GT_EQ:
+		return eval_gt_eq_node(root, fields);
+	case AND:
+		return (evaluate_expression(root->left_child, fields) 
+			&& evaluate_expression(root->right_child, fields));
+	case OR:
+		return (evaluate_expression(root->left_child, fields) 
+			|| evaluate_expression(root->right_child, fields));	
+	}
+	return 0;
 }
 
 void print_expression(node_t *root) {
@@ -69,7 +150,7 @@ void print_expression(node_t *root) {
 			printf("%s) ", root->value.string_literal);
 			break;
 		case INT:
-			printf(" %d) ", root->value.int_literal);
+			printf(" %llu) ", (long long unsigned) root->value.int_literal);
 			break;
 		default:
 			break;
diff --git a/src/expression.h b/src/expression.h
index 1c58f68..7ff5068 100644
--- a/src/expression.h
+++ b/src/expression.h
@@ -1,9 +1,12 @@
 #ifndef ZMAP_TREE_H
 #define ZMAP_TREE_H
 
+#include "fieldset.h"
+
 #include <string.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <stdint.h>
 
 enum operation {
 	GT, LT, EQ, NEQ, AND, OR, LT_EQ, GT_EQ
@@ -21,7 +24,7 @@ struct field_id {
 union node_value {
 	struct field_id field; 
 	char *string_literal;
-	int int_literal;
+	uint64_t int_literal;
 	enum operation op;
 };
 
@@ -40,8 +43,6 @@ node_t* make_string_node(char *literal);
 
 node_t* make_int_node(int literal);
 
-int evaluate_expression(node_t *root);
-
-void print_expression(node_t *root);
+int evaluate_expression(node_t *root, fieldset_t *fields);
 
 #endif /* ZMAP_TREE_H */
\ No newline at end of file
diff --git a/src/filter.c b/src/filter.c
index 19ac3ae..49165f3 100644
--- a/src/filter.c
+++ b/src/filter.c
@@ -4,10 +4,32 @@
 #include "y.tab.h"
 #include "../lib/logger.h"
 
+#include <string.h>
+
 extern int yyparse();
 
 node_t *zfilter;
 
+static int validate_node(node_t *node, fielddefset_t *fields)
+{
+	int i;
+	if (node->type != FIELD) {
+		return 1;
+	}
+
+	for (i = 0; i < MAX_FIELDS; i++) {
+		if (fields->fielddefs[i].name) {
+			if (strcmp(fields->fielddefs[i].name, node->value.field.fieldname) == 0) {
+				node->value.field.index = i;
+				return 1;
+			}
+		}
+	}
+	// Didn't find it
+	return 0;
+
+}
+
 int parse_filter_string(char *filter)
 {
 	YY_BUFFER_STATE buffer_state = yy_scan_string(filter);
@@ -23,4 +45,18 @@ int parse_filter_string(char *filter)
 	printf("%s\n", "");
 	fflush(stdout);
 	return 1;
-}
\ No newline at end of file
+}
+
+int validate_filter(node_t *root, fielddefset_t *fields)
+{
+	int valid;
+	if (!root) {
+		return 1;
+	}
+	valid = validate_node(root, fields);
+	if (!valid) {
+		return 0;
+	}
+	return (validate_filter(root->left_child, fields) && validate_filter(root->right_child, fields));
+
+}
diff --git a/src/filter.h b/src/filter.h
index 4de684c..f42af6d 100644
--- a/src/filter.h
+++ b/src/filter.h
@@ -2,6 +2,7 @@
 #define ZMAP_FILTER_H
 
 #include "expression.h"
+#include "fieldset.h"
 
 struct output_filter {
 	node_t *expression;
@@ -9,4 +10,6 @@ struct output_filter {
 
 int parse_filter_string(char *filter);
 
+int validate_filter(node_t *root, fielddefset_t *fields);
+
 #endif /* ZMAP_FILTER_H */
\ No newline at end of file
diff --git a/src/lexer.c b/src/lexer.c
index 7172b5b..b5be489 100644
--- a/src/lexer.c
+++ b/src/lexer.c
@@ -745,7 +745,7 @@ do_action:	/* This label is used only to access EOF actions. */
 case 1:
 YY_RULE_SETUP
 #line 10 "lexer.l"
-yylval.int_literal = atoi(yytext); return T_NUMBER;
+yylval.int_literal = (uint64_t) atoll(yytext); return T_NUMBER;
 	YY_BREAK
 case 2:
 /* rule 2 can match eol */
diff --git a/src/lexer.l b/src/lexer.l
index 63696b2..0fa3b90 100644
--- a/src/lexer.l
+++ b/src/lexer.l
@@ -7,7 +7,7 @@
 %option noinput
 %option nounput
 %%
-[0-9]+               yylval.int_literal = atoi(yytext); return T_NUMBER;
+[0-9]+               yylval.int_literal = (uint64_t) atoll(yytext); return T_NUMBER;
 \n                   /* Ignore end of line */
 [ \t]+               /* Ignore whitespace */
 !=                   return T_NOT_EQ;
diff --git a/src/zmap.c b/src/zmap.c
index ae30562..e61ae65 100644
--- a/src/zmap.c
+++ b/src/zmap.c
@@ -501,9 +501,15 @@ int main(int argc, char *argv[])
 
 	// Parse and validate the output filter, if any
 	if (args.output_filter_arg) {
+		// Run it through yyparse to build the expression tree
 		if (!parse_filter_string(args.output_filter_arg)) {
 			log_fatal("zmap", "Unable to parse filter expression");
 		}
+
+		// Check the fields used against the fieldset in use
+		if (!validate_filter(zconf.filter.expression, &zconf.fsconf.defs)) {
+			log_fatal("zmap", "Field does not exist");
+		}
 	}
 
 	SET_BOOL(zconf.dryrun, dryrun);